http://vgshorts.com/BBCode/BBCode3.php?ID=193
Here's the trigger named "Selection".
As you can see, I've tried 2 different methods for the "If" statements (since, according to the errors shown just under, the problem is right there): Enabled and Visible Dialog Item.

Here's what happens when I press the "Select_btn" Dialog Item (according to Trigger Debugging Window):

Quote

00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 13)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 13)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 13)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 13)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Trigger Error in 'gt_Selection_Func': Could not get 'property' from parameter in 'DialogControlGetPropertyAsBool' (value: 12)
00:02:55.56 Running gt_Selection_Func (Event: TriggerDebug/EventPlayerDialogControl)

I'm wondering what's wrong.
If you require the map, just ask.

Oh wow... I've found the solution: even though a newly created and showed Dialog Item acts as if it is Visible and Enabled, its property hasn't been set as if it was and thus the Editor wasn't recognizing its state.
That's stupid. >_>