Fix crash on opening folders just pasted into empty folder.

[jeremypw]

Fixes #2495

• Ensure not using invalid TreeRowReference
• Add extra null checks
• DRY getting path from model

To reproduce crash in main:

• Create an empty folder and navigate into it
• In another tab, open a folder containing copyable subfolders
• Copy a folder
• Paste the copy into the empty folder
• Immediately click on the (unselected) pasted folder
• Files crashes with a segfault.

[jeremypw]

Not sure exactly what causes the crash but have improved the code around where the crash occurs (from the backtrace) to avoid derefencing null or using an invalid TreeRowReference.