Merge pull request #6429 from wieslawsoltes/AutoCompleteBoxNullReferenceFix

Check if TextBox.Text is null in AutoCompleteBox

src/Avalonia.Controls/AutoCompleteBox.cs

@@ -2005,7 +2005,7 @@ namespace Avalonia.Controls
             // The TextBox.TextChanged event was not firing immediately and
             // was causing an immediate update, even with wrapping. If there is
             // a selection currently, no update should happen.
-            if (IsTextCompletionEnabled && TextBox != null && TextBoxSelectionLength > 0 && TextBoxSelectionStart != TextBox.Text.Length)
+            if (IsTextCompletionEnabled && TextBox != null && TextBoxSelectionLength > 0 && TextBoxSelectionStart != (TextBox.Text?.Length ?? 0))
             {
                 return;
             }
@@ -2303,7 +2303,7 @@ namespace Avalonia.Controls
             {
                 if (IsTextCompletionEnabled && TextBox != null && userInitiated)
                 {
-                    int currentLength = TextBox.Text.Length;
+                    int currentLength = TextBox.Text?.Length ?? 0;
                     int selectionStart = TextBoxSelectionStart;
                     if (selectionStart == text.Length && selectionStart > _textSelectionStart)
                     {