qt/qrreader(zbar): don't call callback if user closes camera window

- without this, in send tab paytoedit, try_payment_identifier will get called and it will error
df1b9a223c/electrum/gui/qt/paytoedit.py (L153)
- also, in all text fields this used to result in clearing the current text (but now it is kept instead)

electrum/gui/qt/qrreader/__init__.py

@@ -105,6 +105,9 @@ def _scan_qrcode_using_zbar(
     else:
         success = True
         error = ""
+    if data is None:
+        # probably user cancelled
+        success = False
     callback(success, error, data)
 
 

electrum/gui/qt/util.py

@@ -672,7 +672,7 @@ class GenericInputHandler:
     ) -> None:
         if setText is None:
             setText = self.setText
-        def cb(success: bool, error: str, data):
+        def cb(success: bool, error: str, data: Optional[str]):
             if not success:
                 if error:
                     show_error(error)