I have no idea what was going on here, so this is what I did before the computer "restarted because of an error":

1. I used OBS to start recording my screen
2. I was in Creative mode, Peaceful difficulty, gamerules doWeatherCycle and doDaylightCycle set to false. I also had a custom resourcepack that changed red dye, diamond ore, pumpkin blur, splash text, and en_us.json.
3. I grabbed a stack of wolf spawn eggs and put them in my first hotbar slot
4. I put a diamond helmet in my 2nd hotbar slot, diamond chestplate in my 3rd hotbar slot (which gave me the achievement "cover me with diamonds"), then I put the diamond pants in the 4th hotbar slot and the diamond boots in the 5th hotbar slot.
5. The screen turned black for a few seconds, then I got an error that looks like the one attached. (I should give credit to the real owner of the image, so the link is here.)

I don't know if this is just a random error with my computer or a glitch in Minecraft. If it is, please fix it, because, well, isn't 1.15 supposed to be all about bug fixes? Plus, it's a really annoying bug.