[Solution found for similar problem]
Same error msg's here, but ONLY on the 'Edit Store' page.
page displayed the Store fields, but in 'bare' layout, e.g. no Save/Apply/Cancel button, description box could not be seen etc.
however, in joomla 'global configuration/site settings', my 'Default WYSIWYG Editor' was blank (nothing selected, also the dropdown box didn't show anything).
so I activated in joomla 'PlugIn Manager' the 'Editor - TinyMCE 2.0'.
then I selected it in 'global configuration/site settings'.
now, I'm still getting the errors, but the rest of the page looks ok (including save button), so I can do the changes and save it at least.
PHP Warning: Attempt to assign property of non-object in C:\Program Files\wamp\www\mywebsite\libraries\joomla\html\editor.php on line 309
PHP Fatal error: Call to a member function get() on a non-object in C:\Program Files\wamp\www\mywebsite\plugins\editors\tinymce.php on line 57
Note that the last error (line 57) has disappeared since I did the editor changes as described above.
Then, I repeated above, and also enabled the 'Editor - NoEditor' via the Plugin Manager.
(still default editor in global configuration is TinyMCE)
Now, all errors are gone!
Note: I didn't set any specific editor for the user (administrator) via Site/User Manager (Parameters/User Editor). You might want todo this aswell if above doesn't fix your problem (you should be able to select one of the 2 available editors now).
Hope this helps a bit!
