Checking if Var Exists Before Evaluating

I get these notices when I try to send an email:

> Notice: Undefined index: track-clicks in /Users/patrick/Documents/web/projects/ninja-forms/wp-content/plugins/mailgun/includes/wp-mail.php on line 142

> Notice: Undefined index: track-opens in /Users/patrick/Documents/web/projects/ninja-forms/wp-content/plugins/mailgun/includes/wp-mail.php on line 143

I checked to make sure the variable exists before evaluating them and that made them go away. :)