[Taxonomy] Clicking disabled delete button results in NPE

Reproduction path

• Edit a taxonomy document
• Select the root item
• Click the 'delete' button (it looks disabled)

Actual

• a NPE is thrown

Expected

• Nothing happens

Note: clicking the 'Move' button in disabled state still opens the 'move' dialog and logs a 'Undetected JcrItemModel' warning.
Note: the color(s) of the delete icon are not correct. It should not red in disabled state and the cross in the red circle should be white in enabled state.