PC How do I remove "Missing Block"s from the poi editor

[lil_joha]

I followed the conversion of ver. 19 to alpha 20 for blocks in my poi's  but chainlink fencing did not convert. Now I am left with all these blocks with question marks. How do I remove them so I can replace the chainlink fence ?  

Found it    Shift Left Click..  Missingblocks are gone.