Bug. TreeGrid collapsing/expanding functionallity freezes

Good day,

Met one issue with Treegrid component on Vaadin 8+ versions.

We leave in very dinamic world where data is changing very fast and we can't be sure that one query that returned one result will respond with same result after one second.

Problem is simple. If we impementing AbstractHierarchicalDataProvider and query inside hasChildren metrod returned positive number of elements and then when user clicked on arrow to expand this element and fetchChildren returned empty stream, then tree table collapsing/expanding functionallity will be freezed.

Solution to cache data on hasChildren call is inappropriate solution, because we should cache a lot of data in memory, and we are not sure, that even it will be used by customer. And data will not be up-to-date.

Possibly related ticket from different angle

I'm attached zip with demonstration of that issue.
TreeGridDemo.zip

Start by running the attached TreeGridDemo.zip and trace AbstractHierarchicalDataProvider behavior through hasChildren and fetchChildren. Reproduce the case where hasChildren reports children but fetchChildren returns an empty stream, then inspect the TreeGrid expansion and collapse handling. Done means the component remains usable after this data change and the scenario is covered by a regression test.