Fix race condition in TreeSupportBaseCircle which may sometimes cause a crash#2210

rburema merged 3 commits intoUltimaker:mainUltimaker/CuraEngine:mainfrom

ThomasRahm:tree-support-base-circle-race-condition-fixThomasRahm/CuraEngine:tree-support-base-circle-race-condition-fixCopy head branch name to clipboard