breaking the bottom block glitches the sign

[wysohn]

I'm talking about the edit mode

so the sign will just break if the bottom block is broken

yet the sign seems to be still there.

I placed [checkpoint] sign with d 15 trigger, and I accidentally broke the bottom block;

I placed the same sign at the exactly same spot, and now I see two duplicated 'check point reached' message.

need to protect it from breaking or automatically delete the sign info