How to reproduce

On the plugin manager, disable the user's default editor plugin and the system's default editor plugin. If unsure, feel free to disable all the editor plugins.
Open Article Manager to edit an article. This involves the editor and raises the warnings.

Symptoms

Trying to instantiate an editor that is not available, a PHP notice and a PHP warning are generated in the error output:

PHP Notice: Trying to get property of non-object in [...]/libraries/cms/editor/editor.php on line 508
PHP Warning: Attempt to assign property of non-object in [...]/libraries/cms/editor/editor.php on line 510

Cause

This happens because JEditor::_loadEditor() function calls JPluginHelper::getPlugin() without considering its return type.
JPluginHelper::getPlugin() can return either an object or an array (WTF?), while JEditor::_loadEditor() always suppose (ingenuously) to deal with an object.

When no matching plugin is found, an empty array is returned to $plugin;
$plugin = JPluginHelper::getPlugin('editors', $this->_name);

this subsequent code raises the PHP Notice
$params->loadString($plugin->params);

this subsequent code raises the PHP Warning
$plugin->params = $params;

Fix

As the subsequent code needs to deal with a valid object, simply ensure to provide a valid object rather than overcomplicate the code adding further if / else / return statements.