Bug restoring model panel from session

[Greg Couch]

Open an NMR structure, e.g., 176d. Save a session with it. Then open the session. In the results, you'll see:

Error processing trigger "model name changed"
Traceback (most recent call last):
  File ".../ChimeraX.app/lib/python3.6/site-packages/chimerax/core/triggerset.py", line 126, in invoke
    return self._func(self._name, data)
  File ".../ChimeraX.app/lib/python3.6/site-packages/chimerax/model_panel/tool.py", line 141, in _fill_tree
    parent = item_stack[0] if len(item_stack) == 1 else item_stack[len_id-1]
IndexError: list index out of range

[pett]

Fix: don't fire "model name changed" trigger if model not open