Problem In Custom PersistParamater in Grasshopper

Nearly, When I Coding to create an Custom Object with Custom persist paramater. I find an very strange problerm, the strange is that you actually can not find where throw the Exception at the Code. And I set an breakpoint in the "SetFilePath" Component's  SoleInstance method. But the code isn't run into the method before pupup this Component Exception. So I think It is not the plobrem with the "SetFilePath"

Component's plobrem.

And It troubled me for a long time, Could help me find what wrong with code as below, and the problerm sreenshot as below:

Notice:

The paramater is set through "set one path " menu item. and the "set one path" menu item ivoke the method of "Prompt_Plural" in Custom PersistentParam know as "PathParameter" in the below.

Plobrem screenShot:

Code ScreenShot: