FLEX-27509
CAUSE: In an AdvancedDataGrid with non-text item renderers AdvancedDataGrid.expandItem() applies masks onto those item renderers when it needs to close a node. At the same time, AdvancedListBase has its own custom mechanism for using masks to scroll vertically in a more efficient way. In order to detect whether it's used this mechanism, and reset the changes (detection happens in AdvancedListBase.removeClipMask(), while the mechanism is applied in addClipMask()), it simply asks whether the non-text item renderer has a mask applied. However, this mask could have been applied by the expandItem() mechanism mentioned above, which means that addClipMask() will never have been called. As a result, itemMaskFreeList is still null, which leads to the fatal.
SOLUTION: Ideally there should be a flag that specifies whether AdvancedListBase.addClipMask() has applied the masking mechanism. But for now we can simply check that itemMaskFreeList isn't null before we use it.
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