fix: crash when closing qrscanner window (see #3546)

gui/qt/qrtextedit.py

@@ -56,6 +56,8 @@ class ScanQRTextEdit(ButtonsTextEdit, MessageBoxMixin):
         except BaseException as e:
             self.show_error(str(e))
             data = ''
+        if not data:
+            data = ''
         self.setText(data)
         return data