Fix Crashlytics crash: "Null check operator used on a null value" in MessageInputWidget (#1362)

Co-authored-by: kevmo314 <[email protected]>
Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: Kevin Wang <[email protected]>

Author: Copilot

lib/components/message_input.dart

@@ -185,6 +185,9 @@ class _MessageInputWidgetState extends State<MessageInputWidget> {
 
   // Handles any shared data we may receive.
   void _handleSharedData(String sharedData) {
+    if (!mounted || sharedData.isEmpty) {
+      return;
+    }
     setState(() {
       _textEditingController?.text = sharedData;
     });