check we have a presenter before allowing navigation inside textbox.

src/Avalonia.Controls/TextBox.cs

@@ -1098,29 +1098,35 @@ namespace Avalonia.Controls
 
         private bool MoveVertical(int count)
         {
-            var formattedText = _presenter.FormattedText;
-            var lines = formattedText.GetLines().ToList();
-            var caretIndex = CaretIndex;
-            var lineIndex = GetLine(caretIndex, lines) + count;
-
-            if (lineIndex >= 0 && lineIndex < lines.Count)
-            {
-                var line = lines[lineIndex];
-                var rect = formattedText.HitTestTextPosition(caretIndex);
-                var y = count < 0 ? rect.Y : rect.Bottom;
-                var point = new Point(rect.X, y + (count * (line.Height / 2)));
-                var hit = formattedText.HitTestPoint(point);
-                CaretIndex = hit.TextPosition + (hit.IsTrailing ? 1 : 0);
-                return true;
-            }
-            else
+            if (_presenter != null)
             {
-                return false;
+                var formattedText = _presenter.FormattedText;
+                var lines = formattedText.GetLines().ToList();
+                var caretIndex = CaretIndex;
+                var lineIndex = GetLine(caretIndex, lines) + count;
+
+                if (lineIndex >= 0 && lineIndex < lines.Count)
+                {
+                    var line = lines[lineIndex];
+                    var rect = formattedText.HitTestTextPosition(caretIndex);
+                    var y = count < 0 ? rect.Y : rect.Bottom;
+                    var point = new Point(rect.X, y + (count * (line.Height / 2)));
+                    var hit = formattedText.HitTestPoint(point);
+                    CaretIndex = hit.TextPosition + (hit.IsTrailing ? 1 : 0);
+                    return true;
+                }
             }
+
+            return false;
         }
 
         private void MoveHome(bool document)
         {
+            if (_presenter == null)
+            {
+                return;
+            }
+            
             var text = Text ?? string.Empty;
             var caretIndex = CaretIndex;
 
@@ -1151,6 +1157,11 @@ namespace Avalonia.Controls
 
         private void MoveEnd(bool document)
         {
+            if (_presenter == null)
+            {
+                return;
+            }
+            
             var text = Text ?? string.Empty;
             var caretIndex = CaretIndex;