Update CustomCellHandler to check if the cell is not null when handling mouse click

[sergey-me]

Update CustomCellHandler to check if the cell content handler is connected to a cell when handling mouse click.

Otherwise, a null reference exception might get thrown when the user clicks too fast on a cell.

In the following scenario we can get a content that is not connected to a cell:

• a tree has logic expanding/collapsing items on a cell double-click;
• the user clicks too fast on a cell;
• double-click causes the row expanding/collapsing;
• the third click (e.ClickCount > 2) happens when the cells presenter is refreshed;
• at this moment the cell content is disconnected from any cell (ctl?.GetVisualParent<swc.DataGridCell>() returns null).

[cwensley]

Hey @sergey-me, thanks for submitting the fix and especially providing a use case that replicates it! Very much appreciated.