Whenever I'm trying to edit a replay, and I put the little yellow thing at the start (new to replay mod, sorry) it just straight up crashes my game.
Here's the error code:
The game crashed whilst calling gui method
Error: java.lang.NullPointerException: Calling Gui method
Exit Code: -1
If anyone could help that would be wondrous!

Duplicate of #227

Seems to be a rare bug happening to only a few people. As a workaround you can enable Quick Mode (Q by default) and only disable it when you render you video.

Duplicate of #227

Seems to be a rare bug happening to only a few people. As a workaround you can enable Quick Mode (Q by default) and only disable it when you render you video.
But I'm using 1.8.9 and Quick Mode dosen't work in that version.

But I'm using 1.8.9 and Quick Mode dosen't work in that version.

In that case, you'll either have to update to a modern version of Minecraft or just live with the bug (i.e. try to avoid rewinding at all costs) until it is fixed. I'm not aware of any other workaround.