Fix IndexOutOfBoundsException for case when nothing was selected

I've seen an IndexOutOfBounds exception on occasion while using this plugin. The stacktrace points to the line containing: String selectedThemeName = themeSelectionList.getSelection()[0];

It is possible to reach this line without having made a selection, causing the array to be empty. The fix is pretty simple: check that the array contains at least one element and then only act on it if that element is present.