Preferences do not open if no files are open

[koppor]

JabRef version 3.3dev--snapshot--2016-04-06--master--6488cf7 on Windows 10

Steps to reproduce:

1. Close all opened files
2. Open Preferences

Result:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at net.sf.jabref.gui.labelpattern.LabelPatternPanel.buildGUI(LabelPatternPanel.java:107)
    at net.sf.jabref.gui.labelpattern.LabelPatternPanel.<init>(LabelPatternPanel.java:55)
    at net.sf.jabref.gui.preftabs.LabelPatternPrefTab.<init>(LabelPatternPrefTab.java:58)
    at net.sf.jabref.gui.preftabs.PreferencesDialog.<init>(PreferencesDialog.java:97)

[stefan-kolb]

        for (EntryType type : EntryTypes.getAllValues(panel.getBibDatabaseContext().getMode())) {
            textFields.put(type.getName().toLowerCase(), addEntryType(pan, type, y));
            y++;
        }