Fix IDEA-84220, registerShortcutOn(tree) wasn't called after unregisterCustomShortcutSet

If you pin the active call hierarchy (#1) and open a new one (#2) the following things happen:
 #1 unregister shortcut
 #2 register shortcut
switch back to #1
 #2 unregister shortcut
=> no shortcuts are active

Sometimes the shortcut is deactivated right after initializing the call hierarchy => no shortcut is active. I don't know why this is happening.