IDLE test_textview: add comments and test, increase coverage to 100% (#1641)

diff --git a/Lib/idlelib/textview.py b/Lib/idlelib/textview.py
index f33ac32..dc1a7bf 100644
--- a/Lib/idlelib/textview.py
+++ b/Lib/idlelib/textview.py
@@ -87,6 +87,7 @@
                   parent=parent)
     else:
         return view_text(parent, title, contents, modal, _utest=_utest)
+    return None
 
 
 if __name__ == '__main__':