Fix: unsafe operation while getting a overlay entry in text selection

lib/src/editor/widgets/text/text_selection.dart

@@ -428,12 +428,12 @@ class EditorTextSelectionOverlay {
     );
 
     if (builtMagnifier == null) return;
-
+    
     _magnifierController.show(
       context: context,
       below: magnifierConfiguration.shouldDisplayHandlesInMagnifier
           ? null
-          : _handles![0],
+          : _handles?.elementAtOrNull(0),
       builder: (_) => builtMagnifier,
     );
   }