You may or may not run into this popup. For now, the only solution is to point the deopfuscator into the right direction.
Solution: Point it to ~/.gradle/caches/minecraft/net/minecraftforge/forge/1.7.10-10.13.4.1614-1.7.10/unpacked/conf
. On Windows, please use %USERPROFILE%/.gradle/caches/minecraft/net/minecraftforge/forge/1.7.10-10.13.4.1614-1.7.10/unpacked/conf
.