Hopefully fix the random crashes with threads

[santouits]

fixes #21685
possibly other random things too

[bojidar-bg]

Will conflict with #30801, though if it does fix thread-related crashes...

[santouits]

Will conflict with #30801, though if it does fix thread-related crashes...

To be honest I don't know if that check I put there helps with the fix, I just thought that it shouldn't be interacting with the debugger if it is a function running in a thread. The example in the issue stopped crashing after I added the lock in object.h, but I left that change too. I will see if it works without that check.

[santouits]

Alright it fixes the issue without that code. I removed it and it can be added in another PR, I think without it it will mess the debugger if you are on a breakpoint in the main thread and another thread keeps running.

[akien-mga]

For the reference, that's also what #28791 implemented (together with a new method).

Let's give this PR a spin (pun intended) :)

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.1.2.