Weird Error

I'm working on making a (possibly) open-source library of code for RPGs, mostly text RPGs...

Unfortunetly in order for portablity between console and GUI alike I've run into a problem with an "In" function which is essential input() for maxGUI.

Function In:String(Message:String, MainWindow:TGadget, IsChoice:Int = 0, Choices:TList = Null)
	Local Message_Window:TGadget = CreateWindow(Message, 100, 100, 320, 240)
	Local Message_Label:TGadget = CreateLabel(Message, 0, 0, 320, 20, Message_Window)
	Local Message_Button:TGadget = CreateButton("Ok", 130, 150, 60, 20, Message_Window)
	Local Message_ListBox:TGadget = CreateListBox(0, 20, 320, 130, Message_Window)
	HideGadget(Message_ListBox)
	Local Message_TextField:TGadget = CreateTextField(0, 20, 320, 20, Message_Window)
	HideGadget(Message_TextField)
	Local ExitTime:Int = False
	Local Temp_String:String
	
	If IsChoice = True
		ShowGadget(Message_ListBox)

		For Temp_String = EachIn Choices
			AddGadgetItem(Message_ListBox, Temp_String)
		Next
	ElseIf IsChoice = False
		Message_Window.height = 100
		Message_Button.ypos = 20
		ShowGadget(Message_TextField)
	End If
	
	While Not ExitTime = True
		WaitEvent()
		Select EventID()
			Case EVENT_WINDOWCLOSE
				Select EventSource()
					Case Message_Window
						ExitTime = True
					Case MainWindow
						End
				End Select
			Case EVENT_GADGETACTION
				Select EventSource()
					Case Message_Button
						ExitTime = True
				End Select
		End Select
	Wend
	FreeGadget(Message_Window)
	If IsChoice = True
		Print SelectedGadgetItem(Message_ListBox)
		Return GadgetItemText(Message_ListBox, SelectedGadgetItem(Message_ListBox))
	ElseIf IsChoice = False
		Return TextFieldText(Message_TextField)
	End If
End Function

That's the code, but whenever I compile it gives this error:

Unhandled Exception:Invalid Gadget type

Any help/suggestions?