fix TypeError: channel.execCallbacks[message.id] is not a function

we need to avoid calling setHtml() until the bridge command has
had a chance to pass its return code back down the channel

aqt/webview.py

@@ -63,7 +63,8 @@ class AnkiWebPage(QWebEnginePage):
         return False
 
     def _onCmd(self, str):
-        self._onBridgeCmd(str)
+        from aqt import mw
+        mw.progress.timer(1, lambda: self._onBridgeCmd(str), False)
 
 # Main web view
 ##########################################################################