Removing the foreground widget from a tab panel causes it to be hidden

[quigleyj-mavenomics]

When removing the foreground widget from a tab panel, either by adding it to another region or sending it a close-request, the TabPanel will hide the foreground widget.

The culprit is the currentChanged handler in TabPanel, which hide()s the previous foreground widget without checking to see if the previous widget is still a child of the panel.

I have some free time this weekend, so I can submit PRs for this and #362 if you'd like.

[sccolbert]

Thanks for reporting this!

[sccolbert]

Although I wonder if previousTitle should be null for this case. i.e. the true bug might be in the tab bar. Perhaps the previousTitle should be null if it's already been removed.