The MapFrame.htm file in the JavaPost sample is missing a variable that is causing this error to occur.
1. Navigate to the JavaPost directory and open MapFrame.htm in a text editor.
2. Use the Find tool to locate the following text:
var cornerOffset = 8;
3. Insert the following lines below it:
// number of form to use - Generic sample only
var genericLoadForm = 1;
4. Save the file.
After this change, the JavaPost sample starts as expected.