Update toolbar.dart (#55)

fix

lib/widgets/toolbar.dart

@@ -518,6 +518,8 @@ class _ImageButtonState extends State<ImageButton> {
 
   Future<String> _pickImage(ImageSource source) async {
     final PickedFile pickedFile = await _picker.getImage(source: source);
+    if (pickedFile == null) return null;
+    
     final File file = File(pickedFile.path);
 
     if (file == null || widget.onImagePickCallback == null) return null;