My /system/priv-app/DocumentsUI is empty obviously. Putting any 3rd party DocumentsUI.apk in it results in bootloop. I have a custom rom (Aosp extended).
Flashing the /system again would fix the problem I think. Just want to make sure if this will make the condition worse or im going alright?
This answer was before the edit but holds
Flashing /Data would result in:
Losing your user data.
Worse, it's not going to solve your problem.
Solutions
Dirty flash the ROM ( flash /system without wiping /Data)
As suggested by Irfan in comments,
Extract the file from ROM's zip file and put in proper place with proper permissions.
Second option requires a good understanding of Android. For me, dirty flashing would be fastest and simplest.
Q & A