How to reproduce:
1. Open the attached "NoGithubReproProject" repro project
2. Find the "New Folder" folder and open it in your file explorer
3. Delete "New Folder" folder but leave its .meta file intact
4. Go back to the Editor and observe that the folder is missing
5. Unfocus and re-focus the Editor to re-create it
6. Observe the console

Expected result: no errors are thrown when re-creating folders from loose .meta files
Actual result: "Assertion failed on expression: 'itGuid != guidMap.end()'" error is thrown once the folder is re-created

Reproduced in: 2020.1.0a9, 2020.1.0a7

Notes:
-on older versions, the folder would not get re-created from its .meta file and thus no error would be present (Changed by Case 1177654)
-In a new empty project, Unity will just delete the orphaned meta file but if there is a ".hg", ".svn" or ".git" folder in the project root (like in "NoGithubReproProject") or version control is enabled, the code will recreate the folder but cause the assert afterward when refreshing in step 5