Fix an issue that caused an NPE

If the overflow is empty when the last bubble
is dismissed we collapse the stack. If it's not empty
we show the overflow contents.

We were overflowing bubbles AFTER making the choice
to select the overflow or collapse the stack which
causes problems.

Test: atest BubbleDataTest
Test: manual - have 1 bubble and an empty overflow
             - navigate to the overflow, dismiss the bubble
             => no crash
Bug: 202320538
Change-Id: Ic838c5c61bcd714d8de94672d3c4ed753d4f6fa6