When using a UI Action on a related list using a GlideDialogWindow, you may encounter where the dialog window will not fully load
showing an image similar to what you see below with most likely a different title.
There will also be an error thrown in the console similar to this:
Uncaught TypeError: Cannot read property 'getChecked' of undefined
This is happening because the reference to the g_list is being used in the get() method of GlideList2 within the UI Action.
This would look like this: GlideList2.get(g_list);
This will work on lists, but not on related lists.
There is a simple fix to this, which is to instead reference the listID of the g_list.
Depending on what is the need there are many ways of doing this. An example looks like this:
var listID = g_list.listID;
var list = Glidelist2.get(listID);
This will allow the record that the related list is on to perform GlideList2 methods, such as getChecked().