GUI throws NullPointerException upon completion.

[djorlando24]

VICompactor is functionally working on my MacOS setup with Java SE 9.0.4, but occasionally it throws NullPointerException when finished and about to show the "File compacted" dialog box. The compacted VDI file seems fine. I've not been able to find a pattern to it, it seems to happen with big VDI files though (many GB in size).

java.lang.NullPointerException
	at org.pfeifer.extreader.Ext4Bitmap.fillBlockGroupBitmap(Ext4Bitmap.java:65)
	at org.pfeifer.extreader.Ext4Bitmap.adjustBitmap(Ext4Bitmap.java:59)
	at org.pfeifer.extreader.Ext4Bitmap.isBlockInUse(Ext4Bitmap.java:51)
	at org.pfeifer.vdicompactor.ExtAllocationBitmap.isAllocated(ExtAllocationBitmap.java:47)
	at org.pfeifer.vdicompactor.AllocationAwareBlockReader.blockIsZero(AllocationAwareBlockReader.java:99)
	at org.pfeifer.blockreader.ConcatenatedBlockReader.blockIsZero(ConcatenatedBlockReader.java:109)
	at org.pfeifer.vdicompactor.VDICompactor.compactVDI(VDICompactor.java:144)
	at org.pfeifer.vdicompactor.VDICompactor.compactVDI(VDICompactor.java:93)
	at org.pfeifer.vdicompactor.VICompactorFrame$6.doInBackground(VICompactorFrame.java:209)
	at java.desktop/javax.swing.SwingWorker$1.call(SwingWorker.java:295)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.desktop/javax.swing.SwingWorker.run(SwingWorker.java:334)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
	at java.base/java.lang.Thread.run(Thread.java:844)