Make comparing IndexPath with null do something useful.

src/Avalonia.Controls/IndexPath.cs

@@ -58,6 +58,11 @@ namespace Avalonia.Controls
 
         public int GetAt(int index)
         {
+            if (index >= GetSize())
+            {
+                throw new IndexOutOfRangeException();
+            }
+
             return _path?[index] ?? (_index - 1);
         }
 
@@ -169,5 +174,7 @@ namespace Avalonia.Controls
         public static bool operator >=(IndexPath x, IndexPath y) { return x.CompareTo(y) >= 0; }
         public static bool operator ==(IndexPath x, IndexPath y) { return x.CompareTo(y) == 0; }
         public static bool operator !=(IndexPath x, IndexPath y) { return x.CompareTo(y) != 0; }
+        public static bool operator ==(IndexPath? x, IndexPath? y) { return (x ?? default).CompareTo(y ?? default) == 0; }
+        public static bool operator !=(IndexPath? x, IndexPath? y) { return (x ?? default).CompareTo(y ?? default) != 0; }
     }
 }

tests/Avalonia.Controls.UnitTests/IndexPathTests.cs

@@ -77,5 +77,19 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(0, a.CompareTo(b));
             Assert.Equal(a.GetHashCode(), b.GetHashCode());
         }
+
+        [Fact]
+        public void Null_Equality()
+        {
+            var a = new IndexPath(null);
+            var b = new IndexPath(1);
+
+            // Implementing operator == on a struct automatically implements an operator which
+            // accepts null, so make sure this does something useful.
+            Assert.True(a == null);
+            Assert.False(a != null);
+            Assert.False(b == null);
+            Assert.True(b != null);
+        }
     }
 }