Compatibility with Admin Views, action: Modify entity values

I had the same issue with version 7.x-1.9. The issue seems to come from a bad treatment when dealing with a non mandatory field where the "N/A" empty case generates a null.
Here is my fix, it should work for most of the case :

ORIGINAL CODE in modules/term_reference_tree/term_reference_tree.module

function _term_reference_tree_get_options(&$terms, &$allowed, $filter) {
  $options = array();

  if (is_array($terms) && count($terms) > 0) {
    foreach($terms as $term) {
	  if (!$filter || (is_array($allowed) && $allowed[$term->tid])) {   // *** issue as $term->tid is null for the empty option *** 
		$options[$term->tid] = entity_label('taxonomy_term', $term);
		$options += _term_reference_tree_get_options($term->children, $allowed, $filter);
	  }
    }
  }
  return $options;
}

NEW CODE in modules/term_reference_tree/term_reference_tree.module

function _term_reference_tree_get_options(&$terms, &$allowed, $filter) {
  $options = array();

  if (is_array($terms) && count($terms) > 0) {
    foreach($terms as $term) {
		if($term->tid)
		{
		  if (!$filter || (is_array($allowed) && $allowed[$term->tid])) {
			$options[$term->tid] = entity_label('taxonomy_term', $term);
			$options += _term_reference_tree_get_options($term->children, $allowed, $filter);
		  }
		}
		else
		{
			$options[] = entity_label('taxonomy_term', $term);
			$options += _term_reference_tree_get_options($term->children, $allowed, $filter);
		}
    }
  }
  return $options;
}

FranckV, thanks for the suggestion, it is a good starting point. The else statement is extraneous and should still check the first condition in the OR.
Here's a simpler patch against 7.x-1.x-dev that fixes the same issue.

Actually, scrap the above patch, all it needs is an isset() in the original if condition.
Here's another patch.

Updates patch in #3 to apply to 7.x-1.10 and 7.x-1.x dev.

Look good