UI locks up

[chipkent]

I managed to get the UI to lock up with this error. It seemed to be unresponsive after getting to this state. DHE demo system.

[niloc132]

The "lock up" part is probably resolved by deephaven/deephaven-core#5501 (in main, but unreleased, and backported to 0.34.2), but the error message overlay for a dh-ui setup issue could probably be presented in a better way.

[dsmmcken]

The error itself, while presented wrong, does represent an actual error in your code.

"Cannot mix layout and non-layout elements" could be better worded too to describe which children can not be returned in tree provided.