Fix java.lang.IndexOutOfBoundsException when closing a view frame after adding a source.

This bug occurs with ViewerFrames created e.g. in Mastodon. It does not
happen for ViewerFrames opened with BdvFunction.show().

If a source is appended to the viewerframe - e.g. the bounding-box
source - it is not possible to close the viewframe without triggering
the excpetion pasted below.

Somehow, when the frame is closing, the VisibilityAndGroupingDialog
instance goes through the buttons list after it has been cleared and
tries to set the selected state of one.

This ad-hoc fix simply checks that there is a button to set the
state of before doing so.

To see this bug in action, check the branch 'bbox' of the
trndy/mastodon-tracking artifact. Creating a bounding-box during
the detection sequence wizard generates this bug.

Exception in thread "AWT-EventQueue-0" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
	at java.util.ArrayList.rangeCheck(Unknown Source)
	at java.util.ArrayList.get(Unknown Source)
	at bdv.tools.VisibilityAndGroupingDialog$GroupingPanel.update(VisibilityAndGroupingDialog.java:572)
	at bdv.tools.VisibilityAndGroupingDialog$GroupingPanel.visibilityChanged(VisibilityAndGroupingDialog.java:607)
	at bdv.viewer.VisibilityAndGrouping.update(VisibilityAndGrouping.java:375)
	at bdv.viewer.ViewerPanel.removeSource(ViewerPanel.java:370)
	at bdv.tools.boundingbox.TransformedBoxOverlaySource.removeFromViewer(TransformedBoxOverlaySource.java:120)
	at bdv.tools.boundingbox.TransformedBoxEditor.uninstall(TransformedBoxEditor.java:180)
	at org.mastodon.trackmate.ui.wizard.descriptors.BoundingBoxDescriptor.uninstall(BoundingBoxDescriptor.java:80)
	at org.mastodon.trackmate.ui.wizard.descriptors.BoundingBoxDescriptor.access$3(BoundingBoxDescriptor.java:77)
	at org.mastodon.trackmate.ui.wizard.descriptors.BoundingBoxDescriptor$1.windowClosing(BoundingBoxDescriptor.java:185)
	at java.awt.AWTEventMulticaster.windowClosing(Unknown Source)
	at java.awt.AWTEventMulticaster.windowClosing(Unknown Source)
	at java.awt.Window.processWindowEvent(Unknown Source)
	at javax.swing.JFrame.processWindowEvent(Unknown Source)
	at java.awt.Window.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
	at java.awt.EventQueue.access$500(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.awt.EventQueue$3.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.awt.EventQueue$4.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)