See TheLimePixel/Modern-Skyblock-3#283

Easy fix, hopefully; use an instanceof check at https://github.com/TeamPneumatic/pnc-repressurized/blob/master/src/main/java/me/desht/pneumaticcraft/common/semiblock/SemiBlockSpawnerAgitator.java#L70 instead of just checking for non-null.

However we should review all other semiblocks to ensure they can also gracefully handle the tile entity being switched out from under them.