Fixes crash in History Panel when a level is unlinked. (#975)
Fixes crash in History Panel when a level is unlinked. (#975)

close #965 
janisozaur/opentoonz,damies13/opentoonz,lordadamson/opentoonz,janisozaur/opentoonz,damies13/opentoonz,damies13/opentoonz,janisozaur/opentoonz,lordadamson/opentoonz,damies13/opentoonz,janisozaur/opentoonz,lordadamson/opentoonz,lordadamson/opentoonz,lordadamson/opentoonz,janisozaur/opentoonz,damies13/opentoonz