diff --git a/src/Avalonia.Controls/IndexRange.cs b/src/Avalonia.Controls/IndexRange.cs
index f3820c087a..b1a112ab39 100644
--- a/src/Avalonia.Controls/IndexRange.cs
+++ b/src/Avalonia.Controls/IndexRange.cs
@@ -3,9 +3,7 @@
 //
 // Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 
-using System;
-using System.Collections.Generic;
-using System.Text;
+#nullable enable
 
 namespace Avalonia.Controls
 {
diff --git a/src/Avalonia.Controls/SelectedItems.cs b/src/Avalonia.Controls/SelectedItems.cs
index 036788e7b2..af43742670 100644
--- a/src/Avalonia.Controls/SelectedItems.cs
+++ b/src/Avalonia.Controls/SelectedItems.cs
@@ -8,13 +8,14 @@ using System.Collections;
 using System.Collections.Generic;
 using SelectedItemInfo = Avalonia.Controls.SelectionModel.SelectedItemInfo;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     internal class SelectedItems<T> : IReadOnlyList<T>
     {
         private readonly List<SelectedItemInfo> _infos;
         private readonly Func<List<SelectedItemInfo>, int, T> _getAtImpl;
-        private int _totalCount;
 
         public SelectedItems(
             List<SelectedItemInfo> infos,
@@ -29,7 +30,7 @@ namespace Avalonia.Controls
 
                 if (node != null)
                 {
-                    _totalCount += node.SelectedCount;
+                    Count += node.SelectedCount;
                 }
                 else
                 {
@@ -40,11 +41,11 @@ namespace Avalonia.Controls
 
         public T this[int index] => _getAtImpl(_infos, index);
 
-        public int Count => _totalCount;
+        public int Count { get; }
 
         public IEnumerator<T> GetEnumerator()
         {
-            for (var i = 0; i < _totalCount; ++i)
+            for (var i = 0; i < Count; ++i)
             {
                 yield return this[i];
             }
diff --git a/src/Avalonia.Controls/SelectionModel.cs b/src/Avalonia.Controls/SelectionModel.cs
index ea3a09d4e7..34d5f78434 100644
--- a/src/Avalonia.Controls/SelectionModel.cs
+++ b/src/Avalonia.Controls/SelectionModel.cs
@@ -8,20 +8,22 @@ using System.Collections.Generic;
 using System.ComponentModel;
 using Avalonia.Controls.Utils;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     public class SelectionModel : INotifyPropertyChanged, IDisposable
     {
-        private SelectionNode _rootNode;
+        private readonly SelectionNode _rootNode;
         private bool _singleSelect;
-        private IReadOnlyList<IndexPath> _selectedIndicesCached;
-        private IReadOnlyList<object> _selectedItemsCached;
-        private SelectionModelChildrenRequestedEventArgs _childrenRequestedEventArgs;
-        private SelectionModelSelectionChangedEventArgs _selectionChangedEventArgs;
+        private IReadOnlyList<IndexPath>? _selectedIndicesCached;
+        private IReadOnlyList<object?>? _selectedItemsCached;
+        private SelectionModelChildrenRequestedEventArgs? _childrenRequestedEventArgs;
+        private SelectionModelSelectionChangedEventArgs? _selectionChangedEventArgs;
 
-        public event EventHandler<SelectionModelChildrenRequestedEventArgs> ChildrenRequested;
-        public event PropertyChangedEventHandler PropertyChanged;
-        public event EventHandler<SelectionModelSelectionChangedEventArgs> SelectionChanged;
+        public event EventHandler<SelectionModelChildrenRequestedEventArgs>? ChildrenRequested;
+        public event PropertyChangedEventHandler? PropertyChanged;
+        public event EventHandler<SelectionModelSelectionChangedEventArgs>? SelectionChanged;
 
         public SelectionModel()
         {
@@ -29,9 +31,9 @@ namespace Avalonia.Controls
             SharedLeafNode = new SelectionNode(this, null);
         }
 
-        public object Source
+        public object? Source
         {
-            get => _rootNode.Source;
+            get => _rootNode?.Source;
             set
             {
                 ClearSelection(resetAnchor: true, raiseSelectionChanged: false);
@@ -74,10 +76,10 @@ namespace Avalonia.Controls
             {
                 IndexPath anchor = default;
 
-                if (_rootNode.AnchorIndex >= 0)
+                if (_rootNode?.AnchorIndex >= 0)
                 {
                     var path = new List<int>();
-                    var current = _rootNode;
+                    SelectionNode? current = _rootNode;
 
                     while (current?.AnchorIndex >= 0)
                     {
@@ -136,11 +138,11 @@ namespace Avalonia.Controls
             }
         }
 
-        public object SelectedItem
+        public object? SelectedItem
         {
             get
             {
-                object item = null;
+                object? item = null;
                 var selectedItems = SelectedItems;
 
                 if (selectedItems?.Count > 0)
@@ -152,7 +154,7 @@ namespace Avalonia.Controls
             }
         }
 
-        public IReadOnlyList<object> SelectedItems
+        public IReadOnlyList<object?> SelectedItems
         {
             get
             {
@@ -179,12 +181,12 @@ namespace Avalonia.Controls
                     // the selected item at a particular index. This avoid having to create the storage and copying
                     // needed in a dumb vector. This also allows us to expose a tree of selected nodes into an 
                     // easier to consume flat vector view of objects.
-                    var selectedItems = new SelectedItems<object> (
+                    var selectedItems = new SelectedItems<object?> (
                         selectedInfos,
                         (infos, index) =>
                         {
                             var currentIndex = 0;
-                            object item = null;
+                            object? item = null;
 
                             foreach (var info in infos)
                             {
@@ -197,7 +199,7 @@ namespace Avalonia.Controls
                                     if (index >= currentIndex && index < currentIndex + currentCount)
                                     {
                                         var targetIndex = node.SelectedIndices[index - currentIndex];
-                                        item = node.ItemsSourceView.GetAt(targetIndex);
+                                        item = node.ItemsSourceView!.GetAt(targetIndex);
                                         break;
                                     }
 
@@ -289,8 +291,6 @@ namespace Avalonia.Controls
         {
             ClearSelection(resetAnchor: false, raiseSelectionChanged: false);
             _rootNode?.Dispose();
-            _rootNode = null;
-            SharedLeafNode = null;
             _selectedIndicesCached = null;
             _selectedItemsCached = null;
         }
@@ -349,7 +349,7 @@ namespace Avalonia.Controls
         {
             var path = index;
             var isRealized = true;
-            var node = _rootNode;
+            SelectionNode? node = _rootNode;
 
             for (int i = 0; i < path.GetSize() - 1; i++)
             {
@@ -370,11 +370,11 @@ namespace Avalonia.Controls
                 var size = path.GetSize();
                 if (size == 0)
                 {
-                    isSelected = SelectionNode.ConvertToNullableBool(node.EvaluateIsSelectedBasedOnChildrenNodes());
+                    isSelected = SelectionNode.ConvertToNullableBool(node!.EvaluateIsSelectedBasedOnChildrenNodes());
                 }
                 else
                 {
-                    isSelected = node.IsSelectedWithPartial(path.GetAt(size - 1));
+                    isSelected = node!.IsSelectedWithPartial(path.GetAt(size - 1));
                 }
             }
 
@@ -457,9 +457,9 @@ namespace Avalonia.Controls
             OnSelectionChanged();
         }
 
-        internal object ResolvePath(object data, SelectionNode sourceNode)
+        internal object? ResolvePath(object data, SelectionNode sourceNode)
         {
-            object resolved = null;
+            object? resolved = null;
 
             // Raise ChildrenRequested event if there is a handler
             if (ChildrenRequested != null)
@@ -565,7 +565,7 @@ namespace Avalonia.Controls
             }
 
             var childNode = _rootNode.GetAt(groupIndex, realizeChild: true);
-            var selected = childNode.Select(itemIndex, select);
+            var selected = childNode!.Select(itemIndex, select);
             
             if (selected)
             {
@@ -653,7 +653,7 @@ namespace Avalonia.Controls
             var selected = false;
             for (int groupIdx = startGroupIndex; groupIdx <= endGroupIndex; groupIdx++)
             {
-                var groupNode = _rootNode.GetAt(groupIdx, realizeChild: true);
+                var groupNode = _rootNode.GetAt(groupIdx, realizeChild: true)!;
                 int startIndex = groupIdx == startGroupIndex ? startItemIndex : 0;
                 int endIndex = groupIdx == endGroupIndex ? endItemIndex : groupNode.DataCount - 1;
                 selected |= groupNode.SelectRange(new IndexRange(startIndex, endIndex), select);
@@ -688,7 +688,7 @@ namespace Avalonia.Controls
                     if (info.Node.DataCount == 0)
                     {
                         // Select only leaf nodes
-                        info.ParentNode.Select(info.Path.GetAt(info.Path.GetSize() - 1), select);
+                        info.ParentNode!.Select(info.Path.GetAt(info.Path.GetSize() - 1), select);
                     }
                 });
 
diff --git a/src/Avalonia.Controls/SelectionModelChildrenRequestedEventArgs.cs b/src/Avalonia.Controls/SelectionModelChildrenRequestedEventArgs.cs
index c5571b9f74..aa5a9b5cad 100644
--- a/src/Avalonia.Controls/SelectionModelChildrenRequestedEventArgs.cs
+++ b/src/Avalonia.Controls/SelectionModelChildrenRequestedEventArgs.cs
@@ -4,27 +4,53 @@
 // Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 
 using System;
-using System.Collections.Generic;
-using System.Text;
+
+#nullable enable
 
 namespace Avalonia.Controls
 {
     public class SelectionModelChildrenRequestedEventArgs : EventArgs
     {
-        private SelectionNode _sourceNode;
+        private object? _source;
+        private SelectionNode? _sourceNode;
 
         internal SelectionModelChildrenRequestedEventArgs(object source, SelectionNode sourceNode)
         {
-            Initialize(source, sourceNode);
+            _source = source;
+            _sourceNode = sourceNode;
         }
 
-        public object Children { get; set; }
-        public object Source { get; private set; }
-        public IndexPath SourceIndex => _sourceNode.IndexPath;
+        public object? Children { get; set; }
+        
+        public object Source
+        {
+            get
+            {
+                if (_source == null)
+                {
+                    throw new ObjectDisposedException(nameof(SelectionModelChildrenRequestedEventArgs));
+                }
+
+                return _source;
+            }
+        }
+
+        public IndexPath SourceIndex
+        {
+            get
+            {
+                if (_sourceNode == null)
+                {
+                    throw new ObjectDisposedException(nameof(SelectionModelChildrenRequestedEventArgs));
+                }
+
+                return _sourceNode.IndexPath;
+            }
+        }
 
-        internal void Initialize(object source, SelectionNode sourceNode)
+        internal void Initialize(object? source, SelectionNode? sourceNode)
         {
-            Source = source;
+            _source = source;
             _sourceNode = sourceNode;
         }
     }
diff --git a/src/Avalonia.Controls/SelectionModelSelectionChangedEventArgs.cs b/src/Avalonia.Controls/SelectionModelSelectionChangedEventArgs.cs
index 8c9e0343de..c8edc1f8ae 100644
--- a/src/Avalonia.Controls/SelectionModelSelectionChangedEventArgs.cs
+++ b/src/Avalonia.Controls/SelectionModelSelectionChangedEventArgs.cs
@@ -4,8 +4,8 @@
 // Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 
 using System;
-using System.Collections.Generic;
-using System.Text;
+
+#nullable enable
 
 namespace Avalonia.Controls
 {
diff --git a/src/Avalonia.Controls/SelectionNode.cs b/src/Avalonia.Controls/SelectionNode.cs
index f5d93681c4..363eb35b94 100644
--- a/src/Avalonia.Controls/SelectionNode.cs
+++ b/src/Avalonia.Controls/SelectionNode.cs
@@ -8,6 +8,8 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -22,17 +24,14 @@ namespace Avalonia.Controls
     internal class SelectionNode : IDisposable
     {
         private readonly SelectionModel _manager;
-        private readonly List<SelectionNode> _childrenNodes = new List<SelectionNode>();
-        private readonly SelectionNode _parent;
+        private readonly List<SelectionNode?> _childrenNodes = new List<SelectionNode?>();
+        private readonly SelectionNode? _parent;
         private readonly List<IndexRange> _selected = new List<IndexRange>();
-        private object _source;
-        private ItemsSourceView _dataSource;
-        private int _selectedCount;
-        private List<int> _selectedIndicesCached = new List<int>();
+        private readonly List<int> _selectedIndicesCached = new List<int>();
+        private object? _source;
         private bool _selectedIndicesCacheIsValid;
-        private int _realizedChildrenNodeCount;
 
-        public SelectionNode(SelectionModel manager, SelectionNode parent)
+        public SelectionNode(SelectionModel manager, SelectionNode? parent)
         {
             _manager = manager;
             _parent = parent;
@@ -40,7 +39,7 @@ namespace Avalonia.Controls
 
         public int AnchorIndex { get; set; } = -1;
 
-        public object Source
+        public object? Source
         {
             get => _source;
             set
@@ -60,7 +59,7 @@ namespace Avalonia.Controls
                         newDataSource = new ItemsSourceView((IEnumerable)value);
                     }
 
-                    _dataSource = newDataSource;
+                    ItemsSourceView = newDataSource;
 
                     HookupCollectionChangedHandler();
                     OnSelectionChanged();
@@ -68,10 +67,10 @@ namespace Avalonia.Controls
             }
         }
 
-        public ItemsSourceView ItemsSourceView => _dataSource;
-        public int DataCount => _dataSource?.Count ?? 0;
+        public ItemsSourceView? ItemsSourceView { get; private set; }
+        public int DataCount => ItemsSourceView?.Count ?? 0;
         public int ChildrenNodeCount => _childrenNodes.Count;
-        public int RealizedChildrenNodeCount => _realizedChildrenNodeCount;
+        public int RealizedChildrenNodeCount { get; private set; }
 
         public IndexPath IndexPath
         {
@@ -101,17 +100,22 @@ namespace Avalonia.Controls
         // create a bunch of leaf node instances - instead i use the same instance m_leafNode to avoid 
         // an explosion of node objects. However, I'm still creating the m_childrenNodes 
         // collection unfortunately.
-        public SelectionNode GetAt(int index, bool realizeChild)
+        public SelectionNode? GetAt(int index, bool realizeChild)
         {
-            SelectionNode child = null;
+            SelectionNode? child = null;
             
             if (realizeChild)
             {
+                if (ItemsSourceView == null || index < 0 || index >= ItemsSourceView.Count)
+                {
+                    throw new IndexOutOfRangeException();
+                }
+
                 if (_childrenNodes.Count == 0)
                 {
-                    if (_dataSource != null)
+                    if (ItemsSourceView != null)
                     {
-                        for (int i = 0; i < _dataSource.Count; i++)
+                        for (int i = 0; i < ItemsSourceView.Count; i++)
                         {
                             _childrenNodes.Add(null);
                         }
@@ -120,7 +124,7 @@ namespace Avalonia.Controls
 
                 if (_childrenNodes[index] == null)
                 {
-                    var childData = _dataSource.GetAt(index);
+                    var childData = ItemsSourceView!.GetAt(index);
                     
                     if (childData != null)
                     {
@@ -142,7 +146,7 @@ namespace Avalonia.Controls
                     }
 
                     _childrenNodes[index] = child;
-                    _realizedChildrenNodeCount++;
+                    RealizedChildrenNodeCount++;
                 }
                 else
                 {
@@ -160,7 +164,7 @@ namespace Avalonia.Controls
             return child;
         }
 
-        public int SelectedCount => _selectedCount;
+        public int SelectedCount { get; private set; }
 
         public bool IsSelected(int index)
         {
@@ -205,7 +209,7 @@ namespace Avalonia.Controls
         // Null  -> Some descendents are selected and some are not
         public bool? IsSelectedWithPartial(int index)
         {
-            var selectionState = SelectionState.NotSelected;
+            SelectionState selectionState;
 
             if (_childrenNodes.Count == 0 || // no nodes realized
                 _childrenNodes.Count <= index || // target node is not realized 
@@ -221,7 +225,7 @@ namespace Avalonia.Controls
                 // targetNode is a non-leaf node, containing one or many children nodes. Evaluate 
                 // based on children of targetNode.
                 var targetNode = _childrenNodes[index];
-                selectionState = targetNode.EvaluateIsSelectedBasedOnChildrenNodes();
+                selectionState = targetNode!.EvaluateIsSelectedBasedOnChildrenNodes();
             }
 
             return ConvertToNullableBool(selectionState);
@@ -274,7 +278,7 @@ namespace Avalonia.Controls
 
         public void Dispose()
         {
-            _dataSource?.Dispose();
+            ItemsSourceView?.Dispose();
             UnhookCollectionChangedHandler();
         }
 
@@ -290,9 +294,9 @@ namespace Avalonia.Controls
 
         public void SelectAll()
         {
-            if (_dataSource != null)
+            if (ItemsSourceView != null)
             {
-                var size = _dataSource.Count;
+                var size = ItemsSourceView.Count;
                 
                 if (size > 0)
                 {
@@ -324,17 +328,17 @@ namespace Avalonia.Controls
 
         private void HookupCollectionChangedHandler()
         {
-            if (_dataSource != null)
+            if (ItemsSourceView != null)
             {
-                _dataSource.CollectionChanged += OnSourceListChanged;
+                ItemsSourceView.CollectionChanged += OnSourceListChanged;
             }
         }
 
         private void UnhookCollectionChangedHandler()
         {
-            if (_dataSource != null)
+            if (ItemsSourceView != null)
             {
-                _dataSource.CollectionChanged -= OnSourceListChanged;
+                ItemsSourceView.CollectionChanged -= OnSourceListChanged;
             }
         }
 
@@ -353,11 +357,11 @@ namespace Avalonia.Controls
             {
                 if (!IsSelected(i))
                 {
-                    _selectedCount++;
+                    SelectedCount++;
                 }
             }
 
-            if (oldCount != _selectedCount)
+            if (oldCount != SelectedCount)
             {
                 _selected.Add(addRange);
 
@@ -370,18 +374,18 @@ namespace Avalonia.Controls
 
         private void RemoveRange(IndexRange removeRange, bool raiseOnSelectionChanged)
         {
-            int oldCount = _selectedCount;
+            int oldCount = SelectedCount;
 
             // TODO: Prevent overlap of Ranges in _selected (Task 14107720)
             for (int i = removeRange.Begin; i <= removeRange.End; i++)
             {
                 if (IsSelected(i))
                 {
-                    _selectedCount--;
+                    SelectedCount--;
                 }
             }
 
-            if (oldCount != _selectedCount)
+            if (oldCount != SelectedCount)
             {
                 // Build up a both a list of Ranges to remove and ranges to add
                 var toRemove = new List<IndexRange>();
@@ -448,7 +452,7 @@ namespace Avalonia.Controls
                 OnSelectionChanged();
             }
 
-            _selectedCount = 0;
+            SelectedCount = 0;
             AnchorIndex = -1;
 
             // This will throw away all the children SelectionNodes
@@ -576,7 +580,7 @@ namespace Avalonia.Controls
             // Adjust the anchor
             if (AnchorIndex >= index)
             {
-                AnchorIndex = AnchorIndex + count;
+                AnchorIndex += count;
             }
 
             // Check if adding a node invalidated an ancestors
@@ -610,7 +614,7 @@ namespace Avalonia.Controls
             bool selectionInvalidated = false;
             
             // Remove the items from the selection for leaf
-            if (ItemsSourceView.Count > 0)
+            if (ItemsSourceView!.Count > 0)
             {
                 bool isSelected = false;
 
@@ -650,7 +654,7 @@ namespace Avalonia.Controls
                     {
                         if (_childrenNodes[index] != null)
                         {
-                            _realizedChildrenNodeCount--;
+                            RealizedChildrenNodeCount--;
                         }
                         _childrenNodes.RemoveAt(index);
                     }
@@ -659,14 +663,14 @@ namespace Avalonia.Controls
                 //Adjust the anchor
                 if (AnchorIndex >= index)
                 {
-                    AnchorIndex = AnchorIndex - count;
+                    AnchorIndex -= count;
                 }
             }
             else
             {
                 // No more items in the list, clear
                 ClearSelection();
-                _realizedChildrenNodeCount = 0;
+                RealizedChildrenNodeCount = 0;
                 selectionInvalidated = true;
             }
 
@@ -719,7 +723,7 @@ namespace Avalonia.Controls
 
         public SelectionState EvaluateIsSelectedBasedOnChildrenNodes()
         {
-            SelectionState selectionState = SelectionState.NotSelected;
+            var selectionState = SelectionState.NotSelected;
             int realizedChildrenNodeCount = RealizedChildrenNodeCount;
             int selectedCount = SelectedCount;
 
@@ -739,7 +743,6 @@ namespace Avalonia.Controls
                 {
                     // There are child nodes, walk them individually and evaluate based on each child
                     // being selected/not selected or partially selected.
-                    bool isSelected = false;
                     selectedCount = 0;
                     int notSelectedCount = 0;
                     for (int i = 0; i < ChildrenNodeCount; i++)
diff --git a/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs b/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs
index 38b1dde5d7..93102a7b5b 100644
--- a/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs
+++ b/src/Avalonia.Controls/Utils/SelectionTreeHelper.cs
@@ -7,6 +7,8 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 
+#nullable enable
+
 namespace Avalonia.Controls.Utils
 {
     internal static class SelectionTreeHelper
@@ -26,7 +28,7 @@ namespace Avalonia.Controls.Utils
 
                 if (depth < path.GetSize() - 1)
                 {
-                    node = node.GetAt(childIndex, realizeChildren);
+                    node = node.GetAt(childIndex, realizeChildren)!;
                 }
             }
         }
@@ -48,7 +50,7 @@ namespace Avalonia.Controls.Utils
                 int count = realizeChildren ? nextNode.Node.DataCount : nextNode.Node.ChildrenNodeCount;
                 for (int i = count - 1; i >= 0; i--)
                 {
-                    SelectionNode child = nextNode.Node.GetAt(i, realizeChildren);
+                    var child = nextNode.Node.GetAt(i, realizeChildren);
                     var childPath = nextNode.Path.CloneWithChildIndex(i);
                     if (child != null)
                     {
@@ -154,7 +156,7 @@ namespace Avalonia.Controls.Utils
 
         public struct TreeWalkNodeInfo
         {
-            public TreeWalkNodeInfo(SelectionNode node, IndexPath indexPath, SelectionNode parent)
+            public TreeWalkNodeInfo(SelectionNode node, IndexPath indexPath, SelectionNode? parent)
             {
                 node = node ?? throw new ArgumentNullException(nameof(node));
 
@@ -174,7 +176,7 @@ namespace Avalonia.Controls.Utils
 
             public SelectionNode Node { get; }
             public IndexPath Path { get; }
-            public SelectionNode ParentNode { get; }
+            public SelectionNode? ParentNode { get; }
         };
 
     }