Created On:  05 October 2010
With Silk Test Workbench you may see the error message "The XPath expression is invalid.  XPath must not be empty." when using the .exists property on a application dialog window.
This issue is usually seen in a Visual Test after inserting "a property from control" step to the script to select the exists property.
Each object is made up of a class and locator. In the Verify functionality this information is part of the Object section of the property window.
In order to get this to work at the dialog level you need to cut the locator descriptor from the Object Locator line and copy that to the Action locator line. In doing this you get the property st_LastReturnValue and can use that return value in the conditional statement to drive the test.