Don't throw JS errors when opening MWExtensionInspector on the wrong node type

The setup method just assumed this.node was of the right type, and
would just crash if it wasn't. You could select an image, open the
gallery inspector, and it would happily try to inspect it until
crashing because there was no .mw.extsrc attribute.

Bug: 60502
Change-Id: Idf0e9cf7ccaa37e607374ced150db732926a19f8