Fix re-opening GRF/script settings windows not closing drop down windows

Causing assertion failure on drop down item selection

See: #527

src/newgrf_gui.cpp

@@ -183,6 +183,11 @@ struct NewGRFParametersWindow : public Window {
 		this->InvalidateData();
 	}
 
+	~NewGRFParametersWindow()
+	{
+		HideDropDownMenu(this);
+	}
+
 	/**
 	 * Get a dummy parameter-info object with default information.
 	 * @param nr The param number that should be changed.