null check folder in findElement

https://sentry.io/sharelatex-1/sl-web-server-prod/issues/236000085/

services/web/app/coffee/Features/Project/ProjectLocator.coffee

@@ -26,6 +26,8 @@ module.exports = ProjectLocator =
 			element = _.find searchFolder[elementType], (el)-> el?._id+'' == element_id+'' #need to ToString both id's for robustness
 			if !element? && searchFolder.folders? && searchFolder.folders.length != 0
 				_.each searchFolder.folders, (folder, index)->
+					if !folder?
+						return
 					newPath = {}
 					newPath[key] = value for own key,value of path #make a value copy of the string
 					newPath.fileSystem += "/#{folder.name}"

services/web/test/UnitTests/coffee/Project/ProjectLocatorTests.coffee

@@ -17,7 +17,7 @@ file1 = name:"file1", _id:"dsa9lkdsad"
 subSubFile = name:"subSubFile", _id:"d1d2dk"
 subSubDoc = name:"subdoc.txt", _id:"321dmdwi"
 secondSubFolder = name:"secondSubFolder", _id:"dsa3e23", docs:[subSubDoc], fileRefs:[subSubFile], folders:[]
-subFolder = name:"subFolder", _id:"dsadsa93", folders:[secondSubFolder], docs:[], fileRefs:[]
+subFolder = name:"subFolder", _id:"dsadsa93", folders:[secondSubFolder, null], docs:[], fileRefs:[]
 subFolder1 = name:"subFolder1", _id:"123asdjoij"
 
 rootFolder =