feat: try to fix android ime crash.

src/Ursa/Controls/IPv4Box/IPv4BoxInputMethodClient.cs

@@ -1,6 +1,8 @@
 using Avalonia;
 using Avalonia.Controls.Presenters;
 using Avalonia.Input.TextInput;
+using Avalonia.Media.TextFormatting;
+using System.Text;
 
 namespace Ursa.Controls;
 
@@ -11,11 +13,96 @@ public class IPv4BoxInputMethodClient: TextInputMethodClient
     public override bool SupportsPreedit => false;
     public override bool SupportsSurroundingText => true;
 
-    public override string SurroundingText { get; } = null!;
+    public override string SurroundingText
+    {
+		get
+		{
+			if (_presenter is null)
+			{
+				return "";
+			}
 
-    public override Rect CursorRectangle { get; } = new();
-    public override TextSelection Selection { get; set; }
-    public void SetPresenter(TextPresenter? presenter)
+			var lineIndex = _presenter.TextLayout.GetLineIndexFromCharacterIndex(_presenter.CaretIndex, false);
+
+			var textLine = _presenter.TextLayout.TextLines[lineIndex];
+
+			var lineText = GetTextLineText(textLine);
+
+			return lineText;
+		}
+	}
+
+	private static string GetTextLineText(TextLine textLine)
+	{
+		if (textLine.Length == 0)
+		{
+			return string.Empty;
+		}
+
+		var builder = new StringBuilder();
+
+		foreach (var run in textLine.TextRuns)
+		{
+			if (run.Length > 0)
+			{
+#if NET6_0_OR_GREATER
+				builder.Append(run.Text.Span);
+#else
+                    builder.Append(run.Text.Span.ToArray());
+#endif
+			}
+		}
+
+		var lineText = builder.ToString();
+
+		return lineText;
+	}
+
+	public override Rect CursorRectangle { get; } = new();
+	public override TextSelection Selection
+	{
+		get
+		{
+			if (_presenter is null)
+			{
+				return default;
+			}
+
+			var lineIndex = _presenter.TextLayout.GetLineIndexFromCharacterIndex(_presenter.CaretIndex, false);
+
+			var textLine = _presenter.TextLayout.TextLines[lineIndex];
+
+			var lineStart = textLine.FirstTextSourceIndex;
+
+			var selectionStart = Math.Max(0, _presenter.SelectionStart - lineStart);
+
+			var selectionEnd = Math.Max(0, _presenter.SelectionEnd - lineStart);
+
+			return new TextSelection(selectionStart, selectionEnd);
+		}
+		set
+		{
+			if (_presenter is null)
+			{
+				return;
+			}
+
+			var lineIndex = _presenter.TextLayout.GetLineIndexFromCharacterIndex(_presenter.CaretIndex, false);
+
+			var textLine = _presenter.TextLayout.TextLines[lineIndex];
+
+			var lineStart = textLine.FirstTextSourceIndex;
+
+			var selectionStart = lineStart + value.Start;
+			var selectionEnd = lineStart + value.End;
+
+			_presenter.SelectionStart = selectionStart;
+			_presenter.SelectionEnd = selectionEnd;
+
+			RaiseSelectionChanged();
+		}
+	}
+	public void SetPresenter(TextPresenter? presenter)
     {
         _presenter = presenter;
         this.RaiseTextViewVisualChanged();