Wither will not spawn

Put the summary of the bug you're having here

*What I expected to happen was a wither to spawn
Describe what you thought should happen here

*What actually happened was the structure that spawns a wither stayed and no wither boss was created
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. build a wither spawning statue
3. when it is is completed it will remain with no effects