After crashing, material maker doesn't properly show hierarchy view

[fccDunce]

Material Maker version:

any

OS/device including version:

windows 10, gtx 760

Issue description:

Whenever MM crashes to the desktop, restarting it and opening a material file results in the hierarchy view showing only a non-expandable "Ptex" entry- no file name and none of the groups that exist in the document. This issue can be resolved by opening and picking any option in the hierarchy thumbnails dropdown menu, but repeats in the next crash.

Steps to reproduce: Work until MM crashes*. Relaunch MM and open a file.

• Crashes are rather frequent for me. It can occur at any stage of work, but seems to be more likely when making/removing connections while the 3d preview window is active.

[PROMETHIA-27]

Additionally, double-clicking the Ptex before resolving the hierarchy by picking thumbnail options causes a crash.