Undefined index notice

Thanks for catching this 70m. This should be changed now in 7.x-1.x-dev. Please let me know if it works for you.
If you do not want to use a dev release, all I did was change line 80 from:

$block['content'] .= drupal_render(drupal_get_form('admin_notes_block_form', $note));

to: (Notice no period before the equal sign)

$block['content'] = drupal_render(drupal_get_form('admin_notes_block_form', $note));

Please reopen this issue if that does not solve your problem!

I've found that the above condition still persist in 7.x-1.0-beta1, so I applied the recommended edit. This cleared the error message in Firefox, but not IE nor Chrome. I reset it and tested the fix again with the same results. I would like to eliminate the error message, so I'll keep on eye on this thread.

Any chance we could role a beta 2 for this.

Initiate the block to eliminate the error. Add the following after 'global $user;'
$block = "";

Neither #1 nor #5 solved the problem for me.

$form = drupal_get_form('admin_notes_block_form', $note);
$block['content'] = drupal_render($form);
Try this it may solve the error