kivy: scan_qr_non_android: handle user closing window

handle user closing window without scanning a qr code
(i.e. cancelling scan)

electrum/gui/kivy/main_window.py

@@ -444,7 +444,7 @@ class ElectrumWindow(App, Logger):
             self.show_error("invoice error:" + pr.error)
             self.send_screen.do_clear()
 
-    def on_qr(self, data):
+    def on_qr(self, data: str):
         from electrum.bitcoin import is_address
         data = data.strip()
         if is_address(data):
@@ -548,6 +548,7 @@ class ElectrumWindow(App, Logger):
         try:
             video_dev = self.electrum_config.get_video_device()
             data = qrscanner.scan_barcode(video_dev)
+            if data is not None:
                 on_complete(data)
         except UserFacingException as e:
             self.show_error(e)

electrum/qrscanner.py

@@ -26,6 +26,7 @@
 import os
 import sys
 import ctypes
+from typing import Optional
 
 from .util import UserFacingException
 from .i18n import _
@@ -53,7 +54,7 @@ except BaseException as e1:
             _logger.error(f"failed to load zbar. exceptions: {[e1,e2]!r}")
 
 
-def scan_barcode_ctypes(device='', timeout=-1, display=True, threaded=False):
+def scan_barcode_ctypes(device='', timeout=-1, display=True, threaded=False) -> Optional[str]:
     if libzbar is None:
         raise UserFacingException("Cannot start QR scanner: zbar not available.")
     libzbar.zbar_symbol_get_data.restype = ctypes.c_char_p