[3.6] IDLE test_textview: add comments and test, increase coverage to 100% (GH-1641) (#2018)

(cherry picked from commit 295304d412700cc6621bb592109fa42249a9dcdb)
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__':