fix: designer stops working

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Views/MergeConflictEditor.axaml.cs

@@ -269,6 +269,9 @@ private void OnTextViewPointerWheelChanged(object sender, PointerWheelEventArgs
 
         private void OnTextViewVisualLinesChanged(object sender, EventArgs e)
         {
+            if (Design.IsDesignMode)
+                return;
+
             if (!TextArea.TextView.VisualLinesValid)
             {
                 SetCurrentValue(DisplayRangeProperty, null);