[patch][cla] Grid: Focus manager throws exception when updating structure

[avoidscorn]

In certain situations, dojox.grid._FocusManager keeps a reference to a destroyed view, causing it to throw an exception when it tries to focus the view's header nodes.

The following steps reproduce this bug:

1. Ensure that the grid has enough rows to require a scrollbar. This ensures that the focus manager's focusView attribute is set.
2. Change the grid's query such that the grid no longer needs a scrollbar. This is necessary to ensure that dojox.grid._ViewManager.getFirstScrollingView returns null.
3. Set the grid's structure attribute (it does not matter what the new structure is). Due to the logic in dojox.grid._FocusManager.initFocusView, the focusView attribute will still reference the old, destroyed view, because there is no other scrolling view to use instead.
4. The focus manager causes a "node is undefined" exception to be thrown in its _isHeaderHidden method, because its view's headerNode is undefined at this point.

The attached patch changes initFocusView to check if its existing focusView is destroyed before reusing it. I'm not sure if this is the best place to handle it, but it works for me.

[Nathan Toone]

Unassigning my tickets.

[Stefan Bird]

This ticket no longer has an owner - can this patch be committed?

The problem fixed is still present in 1.5.

[dante]

bryan/evan, can you take a look at this?

[evan]

duplicate with #11280, already fixed with [23618]