Check for the tinymce instance before accessing it (Bug #1365224)

So it will not break responsive design on mobile devices that can't
handle tinyMCE

Note: this does not fix the issues in the admin section for editing
sitepages as I figure that admins should be not editing the admin
section in a moble device.

Change-Id: If4eb087114bb2b7242982e60f790d2a4d24932dc
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Check for the tinymce instance before accessing it (Bug #1365224)"