While testing IGBF-621 I noticed that a null pointer exception was being generated when reopening a file that was just deleted. This is seen in files that were loaded into a custom genome or an "incorrect" genome after they are deleted.

To reproduce:

1) Drag a file into IGB to open the file in a custom genome; alternatively, you could open a file into an "incorrect" genome (such as opening a human genome file into the arabadopsis genome).
2) Delete the file that was just loaded.
3) Attempt to reopen the file by dragging and dropping into IGB.

*Observe: A null pointer exception is generated (see below)

14:37:38.956 ERROR c.a.genometry.thread.CThreadWorker - null
java.lang.NullPointerException: null
at com.affymetrix.genometry.util.SeqUtils.addInvertChildren(SeqUtils.java:262) ~[na:na]
at com.affymetrix.genometry.util.SeqUtils.inverse(SeqUtils.java:241) ~[na:na]
at com.affymetrix.igb.view.factories.AbstractTierGlyph.addMiddleGlyphs(AbstractTierGlyph.java:161) ~[na:na]
at com.affymetrix.igb.view.factories.AbstractTierGlyph.initUnloaded(AbstractTierGlyph.java:145) ~[na:na]
at com.affymetrix.igb.view.TrackView.addTierFor(TrackView.java:256) ~[na:na]
at com.affymetrix.igb.view.TrackView.addEmptyTierFor(TrackView.java:231) ~[na:na]
at com.affymetrix.igb.view.load.GeneralLoadView$3.runInBackground(GeneralLoadView.java:538) ~[na:na]
at com.affymetrix.genometry.thread.CThreadWorker.doInBackground(CThreadWorker.java:73) ~[na:na]
at javax.swing.SwingWorker$1.call(Unknown Source) [na:1.8.0_40]
at java.util.concurrent.FutureTask.run(Unknown Source) [na:1.8.0_40]
at javax.swing.SwingWorker.run(Unknown Source) [na:1.8.0_40]
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [na:1.8.0_40]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [na:1.8.0_40]
at java.lang.Thread.run(Unknown Source) [na:1.8.0_40]