Exception when opening library from File History Menu

[Ka0o0]

• I have tested the latest development version from http://builds.jabref.org/master/ and the problem persists

Opening a library from File -> Recent libraries throws an exception.

Steps to reproduce the behavior:

1. File -> Recent libraries
2. Select a valid one
3. Exception is shown

This is because a immutable SingletonList is created and then passed to OpenDatabaseAction::openFiles