"Update Thumbnail" crashes

[leonard7e]

Material Maker version: https://github.com/RodZill4/material-maker/actions/runs/608556808

OS/device including version: Ubuntu Linux 20.04 GPU: RTX 2060s

Issue description: If I do (Right mouse button in custom library item) -> "Update Thumbnail" MaterialMaker crashes immediately

The logs

leonard@LuckDragon ~> ./app/material_maker_0_94_linux/material_maker.x86_64
Godot Engine v3.2.3.stable.official - https://godotengine.org
OpenGL ES 3.0 Renderer: GeForce RTX 2060 SUPER/PCIe/SSE2
 
WARNING: load: This method has been deprecated and will be removed in the future.
   At: scene/resources/texture.cpp:239.
WARNING: load_image: Ignoring unsupported header information in HDR: GAMMA=1.
   At: modules/hdr/image_loader_hdr.cpp:50.
WARNING: load_image: Ignoring unsupported header information in HDR: PRIMARIES=0 0 0 0 0 0 0 0.
   At: modules/hdr/image_loader_hdr.cpp:50.
ERROR: connect: Condition "!signal_is_valid" is true. Returned: ERR_INVALID_PARAMETER
   At: core/object.cpp:1503.
fish: “./app/material_maker_0_94_linux…” terminated by signal SIGSEGV (Address boundary error)

Steps to reproduce: Do (Right mouse button in custom library item) -> "Update Thumbnail" on a custom library item