Non-existing shader crashes Game Studio

[WithBlueHorns]

Release Type: Official Release

Version: 4.2.0.2122

Describe the bug
In a material, if you try to set shader that doesn't exist, wait for it to build the effect, then do try to do it again with another shader that doesn't exist Game Studio crashes, not always crash windows pops up.
I'm not sure what and why it happens, sometimes you could use the same bad name, sometimes you just do 2 then it works
To Reproduce
Steps to reproduce the behavior:

1. Create a new project
2. Look for "Sphere Material"/"Ground Material"/Create a new material, then in any property of material (ex. diffuse) set the map to "Shader"
3. Type any name of a shader that doesn't exist, followed by pressing enter (could be a few times)
4. Change the name in the field (ex. "test" -> "test2"), wait for the effects from before to build, then click enter, as soon as you do it the Game Studio crashes

Expected behavior
To just give that red blinking effect or really anything other than crashing

Screenshots

crash0001-1076.mp4

Video instead

Log and callstacks
log2.txt
log1.txt

Additional context
Why do I always find such obscure bugs

[Doprez]

So I took a bit of time to look into this issue and the problem seems to come from misuse of multithreading than the shader being not found.

Its a bit hard to recreate in source but I think it would explain why it only seems to happen after changing the shader and hitting enter multiple times. This could also bhe related to the save error that people have run into #1917.