Avoid IndexOutOfBoundsException if dataSources is 0 length

Hi guys,

I was hitting an `IndexOutOfBoundsException` due to trying to index a 0-length `ArrayList` while working on our Jython scripting stuff.  I figure it's OK to just return null if `dataSources` is 0-length since that's what happens anyway, if `dataSources.get(0)` isn't an `AddeImageDataSource`.

Thanks!
Mike