Hello,
I believe this may be a bug, so I'll throw it here in case it helps someone else.
1. The behavior I unexpectedly encountered - When importing an 8.1 project over to 8.3 I noticed my Menu Tree was blank. I originally thought it was an importing issue since I see a 
(Exclamation triangle) over the Session Events after migrating.
2. The behavior I was expecting to see - I expected to see my menu tree on the left-hand side of my screen, but it appeared blank.
3. Steps I took to discover / an alternative solution -
- Made sure I imported the gateway backup as recommended, along with updating browser and trying to do just a project export as well.
- Went into menu tree and was copying and pasting elements until I got to alignItems, I discovered my previous setting "Flex Start" now made the menu tree blank.
- The solution to this was to set it to "Stretch" which so far produced the same behavior from the original 8.1 project (so far.)
4. Versioning:
- The Ignition version number: 8.3.6
- The browser: Microsoft Edge (I know...)
- The designer version: 1.3.6
The switch to "Stretch" seems to have made everything work how I had it before, if this changes, I'll update this forum. But I'm not sure why that needed to be changed between versions.