NPE for uriString

Added an error in multiple places when we suspect uriOfCollectionString is null, to find bugs easier in the future. Not fixed yet.

Also think of other places next (sheetmusic, ...) next.

better stacktrace now: https://www.bugsense.com/dashboard/project/98c9e403/errors/1293148243

only in MusicCD- and Book-ItemScanAddTask.

java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:200)
at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:274)
at java.util.concurrent.FutureTask.setException(FutureTask.java:125)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:308)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
at java.lang.Thread.run(Thread.java:1019)Caused by: java.lang.IllegalArgumentException: uriOfCollectionString is null in doInBackground. (lp:1331590)
at net.lp.collectionista.util.tasks.BookItemScanAddTask.net.lp.collectionista.domain.Product doInBackground(java.lang.String[])(Collectionista:1)
at net.lp.collectionista.util.tasks.ScanAddTask.java.lang.Object doInBackground(java.lang.Object[])(Collectionista:1)
at android.os.AsyncTask$2.call(AsyncTask.java:185)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306)
... 4 more

It is null because it is not restored with the task in restart(). Fixed that. Also improved existsProduct().

No other task have same problem.

Was under radar because of inspection used with restart. Should always save and restore all fields of any task child class.

TODO still: improve NPE checks or remove, when fix is seen to be working completely.