It's been a very long time since I've used this,
It could be that it can't work like this.
Do you have a 'gadget library' assigned?
I would also double check all your permissions as it can be tricky for dashboards
Yes, I've already assigned 'gadget library' into a Dashboard.
Dashboard work perfectly when I use it as standalone Mashup - picture below, but only when using it as a pop up I got that bug.
Unfortunately that probably means it can't work the way you are trying to use it.
You could with some extra work perhaps do it 'manually' all the services etc. are accessible so you could perhaps add a button to your dashboard (header/side) and use the on click to launch the library yourself with a navigation widget and then add the selected gadget using a service.
Never really done it myself, it has been ages since I last worked with dashboards.
Which version of ThingWorx are you running? It appears to be a very old release.
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