"Assign...to selected" button is enabled when no component or invalid component is selected

Found issue:

"Assign current composition to selected item" button seems to be enabled even when nothing is selected or when some invalid component is selected. Would expect it to be enabled only when valid component is selected.

Steps to reproduce:

• Create a 3D project
• Create and save effect with Effect Composer
• Click somewhere in empty space of Navigator so that no component is selected
• ->"Assign...to selected item" button is enabled, which is not expected. Clicking it does not seem to do anything though
• Select some invalid item, e.g. cube and assign
• -> nothing happens
•  Select materiallibrary  in Navigator (toggle "Show only visible items" if not visible) and assign
• -> effect is assigned to materiallibrary. Does not seem to cause issues though.