Displays the form. The method creates and shows the modal or modeless window. Be sure to set the movie property to a valid value, or the window cannot be created and nothing will be displayed on the screen otherwise.
If the method displays a modeless window, the method will return immediately. If the method displays a modal window, the method won't return until the window has been closed.
For modeless form, it will always return null. For modal form, it will return a string. The string can be returned by the ffish_ret command or the close method. If the window is closed by the x button on the top right corner of the form, it will return a empty string.