Preferences and Feature settings panel panel doesn't open

Problem reported by Michael Wilson via email. Original symptom - no DAS data sources available.

DAS registry URL appears to be corrupted - possibly due to the original URL being incorrectly upgraded in the 2.7 -> 2.8 jump from the biojava das client to JDAS.

When asked to open the preferences and fix the problem, the following errors were raised on the console, and the preferences didn't open:

Exception in thread "AWT-EventQueue-0"
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
    at java.lang.String.substring(String.java:1937)
    at
jalview.ws.dbsources.das.datamodel.DasSourceRegistry.addLocalDasSources(DasS
ourceRegistry.java:196)
    at
jalview.ws.dbsources.das.datamodel.DasSourceRegistry.appendLocalSources(DasS
ourceRegistry.java:206)
    at
jalview.ws.dbsources.das.datamodel.DasSourceRegistry.getSources(DasSourceReg
istry.java:168)
    at jalview.gui.DasSourceBrowser.<init>(DasSourceBrowser.java:103)
    at jalview.gui.DasSourceBrowser.<init>(DasSourceBrowser.java:115)
    at jalview.gui.Preferences.<init>(Preferences.java:110)
    at jalview.gui.Desktop.preferences_actionPerformed(Desktop.java:1356)
    at jalview.jbgui.GDesktop$7.actionPerformed(GDesktop.java:198)
    at
javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
    at