I was having a similar issue except my 'Preview' was dithered in DS. I looked in the latest DS user guide that I could find and it appears you have to preview them in Mingle now. I didn't have a lawform as it describes so couldn't get any further and haven't had time to dig deeper. Below is what the user guide says:
Previewing custom forms and pages created in Lawson Design Studio This topic describes how you can preview how a custom form or page you are developing will look to users. Previewing is done in Lawson for Infor Ming.le. These instructions assume you have both Lawson for Infor Ming.le and Design Studio open. Previewing a custom form 1 In Design Studio, open the form customization you want to preview. 2 Open the properties pane on the right side of the screen. To open the properties pane, click somewhere in the white space of the main pane. 3 Click the Custom button. 4 On the More Document Properties pop-up dialog box, move into the Custom ID field and copy the ID to the clipboard. 5 In Lawson for Infor Ming.le, launch the lawform program. In the toolbar search box, type lawform 6 On the lawform screen, populate the following fields: • Token: Type the ID of the form that your customization is based on. For example, if you are previewing a customization for PO20.1, type "po20.1" in this field. • Data Area: Type the name of the data area