Had a quick google, came across the same issue on stackoverflow, They suggested the following:
Replace the line:
$resultTest = $plugin->onDisplay($editor);
which is in line 261* after the line:
// Try to authenticate — only add to array if authentication is successful
Replace it with:
if (method_exists($plugin, ‘onDisplay’)) {
$result[] = $plugin->onDisplay($editor);
}
This code won’t get rid of the problem, but will remove the symptoms. As CrocoDillon points out, the error basically means there is a missing/invalid plugin.