Toggling between mode after deleting the last annotation lead to errors.

[jcfr]

Environment

• OS: Any
• Version: https://github.com/BICCN/cell-locator/commit/acf3fa31eb30358c724fec71c86a534cb7799a05

Description

Toggling between mode after deleting the last annotation lead to errors.

Expected behavior

After deleting the last annotation, the interaction should be set to Explore

How to Reproduce

• Start cell locator
• Select either MNI or CCF
• Click on "+" button (Place point)

• Add few points, right click
• Delete current annotation cilcking on remove button
• Selecting any of the mode (Explore, Edit, Place point ) leads to the following error:

• Traceback (most recent call last):
  File "/path/to/cell-locator-RelWithDebInfo-build/Slicer-build/lib/CellLocator-4.13/qt-scripted-modules/Home.py", line 1592, in handler
  self.setInteractionState(state)
  File "/path/to/cell-locator-RelWithDebInfo-build/Slicer-build/lib/CellLocator-4.13/qt-scripted-modules/Home.py", line 1539, in setInteractionState
  markup = self.Annotations.current.markup
  AttributeError: 'NoneType' object has no attribute 'markup'