Don't crash if we can't get an element's value for editing

Fixes #2150

qutebrowser/browser/commands.py

@@ -1484,6 +1484,10 @@ class CommandDispatcher:
             return
 
         text = elem.value()
+        if text is None:
+            message.error("Could not get text from the focused element.")
+            return
+
         ed = editor.ExternalEditor(self._tabbed_browser)
         ed.editing_finished.connect(functools.partial(
             self.on_editing_finished, elem))

qutebrowser/browser/webelem.py

@@ -139,7 +139,7 @@ class AbstractWebElement(collections.abc.MutableMapping):
         raise NotImplementedError
 
     def value(self):
-        """Get the value attribute for this element."""
+        """Get the value attribute for this element, or None."""
         raise NotImplementedError
 
     def set_value(self, value):

qutebrowser/browser/webengine/webengineelem.py

@@ -91,7 +91,7 @@ class WebEngineElement(webelem.AbstractWebElement):
         return self._js_dict['outer_xml']
 
     def value(self):
-        return self._js_dict['value']
+        return self._js_dict.get('value', None)
 
     def set_value(self, value):
         js_code = javascript.assemble('webelem', 'set_value', self._id, value)