8.9.0: Grid hovering over open editor in unbuffered mode throws IllegalStateException

[gatanaso]

Description
Vaadin Grid with an opened editor in un-buffered mode throws exceptions when row being edited is out of view and hovered:

Steps to reproduce

• Click on the first row to open the editor
• Start scrolling down
• Then hover over the opened row in the editor
• Multiple exceptions are throws

The issue originates from (there is no check about the row or any exception handling):

framework/client/src/main/java/com/vaadin/client/widgets/Grid.java

Line 7777 in 8daef97

TableCellElement cellElement = container

Minimal reproducible example
Here is an application that reproduces this behavior:
myapplication.zip

Expected behavior
The component handles this scenario and no exceptions thrown in the browser console.

Actual behavior
Multiple exceptions are being thrown and logged in the browser console.

[gatanaso]

This issue is somewhat related to #11463