Dashboard: Query Variable editor broken when a plugin implements a broken variable query editor

### What happened? If a datasource plugin has a bad implementation of a query variable plugin editor, and it is the default datasource, the interface to add query variables will break and will refuse to show any datasource selector or editor until either the datasource connection is removed or the connection is not longer the default. ### What did you expect to happen? If a datasource plugin has a broken query editor, I should still be able to change the datasource for a query editor. ### Did this work before? Probably not ### How do we reproduce it? 1. Install a datasource plugin with a broken query variable editor. You can see this example https://github.com/academo/grafana-broken-datasource-queryeditor-test [1] 2. Add a new connection for this datasource, set it as default 3. Try to add a Query Variable 4. Observe you can't do anything in the UI and a JS error. No error is presented. [1] To install this broken datasource: * Clone the repo inside your grafana plugins * Run `npm install` * Run `npm run build` * Restart your grafana development instance ### Is the bug inside a dashboard panel? No ### Environment (with versions)? Grafana: Latest from main. HEAD when reported: 0bd009fb53040e9a4fdef010526d08ab4e75bb18 OS: Irrelevant Browser: Chromium ### Grafana platform? Other ### Datasource(s)? Custom test datasource