Lepiter navigation broken after exception on specific page

[asampal]

On a brand new GT, version 1.0.977, select the page Working with Python source files in the Pages view of the GT book. This causes the debugger to pop up. After this, selecting any other page in the book only changes the title of the page displayed in the right-hand view to the newly selected page's title, but the contents remain unchanged.

[girba]

There are two problems here:

• we should not get an exception for selecting a page in the first place
• we should not break lepiter if we do

[j-brant]

I added a fix for the initial exception.