Steps to reproduce the issue

Create a simple Editor XTD Plugin and call a page where an editor is displayed (like com_content edit)

class PlgButtonSample extends JPlugin
{
       public function onDisplay($name, $asset, $author)
    {
           // Can be also a string or boolean, it always breaks the jform
            return;
        }
}

Expected result

Nothing should be displayed

Actual result

An exception is thrown: (on getInput() where the plugins are loaded)

Call to a member function get() on null

System information (as much as possible)

Staging, php7

Additional comments

@wilsonge This is a regression and 3.5 blocker, as it is going to break any old editor plugins which does not return an array or object..

Probably related to b50885b