JabRef version 3.6 on Mac OS X El Capitan

Steps to reproduce:

1. Open web search
2. Enter "Cook PA" (with or without quotes)
3. It says "Number of references 105. Number to fetch? [click OK]
4. The blue progress bar fills up, but the references never appear

Sometimes I can run a query and some references will appear, but then it hangs. For example, if I search "Tustison [au]" (no quotes), I get some results but then it hangs at about 25% progress. The hangups are accompanied by Exceptions in the console window:

Exception in thread "JabRef CachedThreadPool" java.lang.NullPointerException
    at net.sf.jabref.logic.importer.fileformat.MedlineImporter.addArticleInformation(MedlineImporter.java:516)
    at net.sf.jabref.logic.importer.fileformat.MedlineImporter.parseArticle(MedlineImporter.java:341)
    at net.sf.jabref.logic.importer.fileformat.MedlineImporter.importDatabase(MedlineImporter.java:151)
    at net.sf.jabref.gui.importer.fetcher.MedlineFetcher.fetchMedline(MedlineFetcher.java:226)
    at net.sf.jabref.gui.importer.fetcher.MedlineFetcher.processQuery(MedlineFetcher.java:199)
    at net.sf.jabref.gui.importer.fetcher.GeneralFetcher.lambda$actionPerformed$5(GeneralFetcher.java:223)
    at net.sf.jabref.gui.importer.fetcher.GeneralFetcher$$Lambda$191/1683863109.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

IEEE and Springer search don't seem to have this problem.

I can get specific things from Medline, for example if I enter a single PMID I can get the result. But larger searches sometimes fail with the above error. I had thought it was due to Medline refusing large requests, but I can enter some very general terms (eg searching my last name without initials) and get hundreds of results each time.

I tried the same thing on 3.0 and it works correctly, I can search "Cook PA [au]" and the results show up as expected.