How to access variables in a fieldset with variable_get
Since you are setting #tree to TRUE, the values will be saved in the bi_mx_enhancement_government_ws persistent variable, which will contain an array. For the code you are showing, its value will be similar to the following one.

array(
  'base_url' =&gt; 'https://'
);


Instead of variable_get('base_url', 'https://'), consider using the following code.

$values = variable_get('bi_mx_enhancement_government_ws', array());
$ws_base_url = $values['base_url'];


When you are now using variable_get('base_url', 'https://'), you should use $ws_base_url.

Since you seem to just use a single setting for that fieldset, I would rather use '#tree' =&gt; FALSE, if that form doesn't have any other form field which is using base_url as identifier.