Fix crashe with no current tab in _run_userscript

Not sure why this was a FIXME instead of handling it properly in the first place...

doc/changelog.asciidoc

@@ -118,6 +118,7 @@ Fixed
   when only the text matches.
 - The `progress` and `backforward` statusbar widgets now stay removed if you
   choose to remove them. Previously they would appear again on navigation.
+- Rare crash when running userscripts with crashed renderer processes.
 
 [[v2.5.3]]
 v2.5.3 (unreleased)

qutebrowser/browser/commands.py

@@ -1191,8 +1191,9 @@ class CommandDispatcher:
             env['QUTE_TAB_INDEX'] = str(idx + 1)
             env['QUTE_TITLE'] = self._tabbed_browser.widget.page_title(idx)
 
-        # FIXME:qtwebengine: If tab is None, run_async will fail!
         tab = self._tabbed_browser.widget.currentWidget()
+        if tab is None:
+            raise cmdutils.CommandError("No current tab!")
 
         try:
             url = self._tabbed_browser.current_url()