Current App Id is Updated before Previous App is Unmounted

[ThomThomson]

Problem
App id changes are emitted during navigateToApp - not after the previous app is unmounted. This has the potential to result in undefined behaviour.

Example
The Session service in the data plugin subscribes to appId changes in order to check that there are no active sessions during a navigation. If there is an open session, it throws a fatal error. In the new, deangularized, dashboard plugin the sessions were cleared in a useEffect cleanup function.

On an app navigation, the appId is updated before the previous app is finished unmounting, which results in a fatal error - even though the sessions are cleared immediately afterwards.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[joshdover]

Since currentAppId$ was only intended for consumption by reporting services like Telemetry, this usage is a bit unintended. I'm going to remove the bug label, but we could also consider just closing this if @ThomThomson you were able to find a workaround.

[ThomThomson]

I was able to work around this issue by cleaning up the active session here:

kibana/src/plugins/dashboard/public/application/dashboard_router.tsx

Line 232 in 53f4b21

dataStart.search.session.clear();

We can close this, but it does seem like a natural DX to want to use currentAppId$ in this sort of way, so it might be a good idea to mention in the contract that this is not intended. @Dosant FYI