hide/show DICOM SEG gives AttributeError: 'NoneType' object has no attribute 'redraw_needed'

[Elaine Meng]

Platform: Darwin-17.7.0-x86_64-i386-64bit
ChimeraX Version: 0.9 (2019-03-07)

Description
With the dataset that includes a CT scan and associated SEG models, showing/hiding the SEG models (e.g. with checkboxes in the Model Panel) gives a traceback. Example dataset
https://www.rbvi.ucsf.edu/chimerax/data/dicom/11-06-2014-1-96508-subset.zip

Log:

open /Users/meng/Desktop/11*subset* format dicom

Opened multi-channel map CHEST CT 4, 4 channels, grid size 512,512,236, pixel
0.576,0.576,1.25, shown at step 1, values int16

show #!1.1.1.1.4 models

Exception ignored in: <function Drawing.del at 0x10c5e9d90>
Traceback (most recent call last):
File
"/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-
packages/chimerax/core/graphics/drawing.py", line 1086, in del
self.delete()
File
"/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-
packages/chimerax/map/image3d.py", line 618, in delete
ir._remove_planes()
File
"/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-
packages/chimerax/map/image3d.py", line 532, in _remove_planes
self._drawing.redraw_needed()
AttributeError: 'NoneType' object has no attribute 'redraw_needed'

OpenGL version: 4.1 NVIDIA-10.32.0 355.11.10.10.40.102
OpenGL renderer: NVIDIA GeForce GTX 675MX OpenGL Engine
OpenGL vendor: NVIDIA Corporation

[Tom Goddard]

Fixed.

Error happened when number of blended dicom images changed.