The game crashed whilst rendering overlay
Error: m: Non [a-z0-9/._-] character in path of location: minecraft:item/Glass_Pane

I had accidentally saved this file with capital letters in it, and after creating this model file:
{"parent": "item/generated","textures":{"layer0": "item/Glass_Pane"}}
The game crashed. According to the error message's regex apparently capital letters aren't allowed.