Just a quick note for those possibly searching for ComboBox AS3 and “Cannot access a property or method of a null object reference.” When using the AS3 ComboBox, the above error will be thrown when the ComboBox loses focus, if the ComboBox is in its open state and it has lost reference to the stage. In other words, if you have a UI pane that contains the ComboBox, and you are removing the pane, make sure to first call #close() on all instances of the ComboBox, then remove the container from its parent container. (This can be replicated by clicking open the ComboBox, then clicking on the stage.) Unfortunately, the isOpen property for the ComboBox is not publicly accessible, but the #close() method is a NOOP if it is already closed, so just making sure to close it is no big deal. I ran into this error because I was listening for a stage focus event which triggered the immediate removal of the container pane in question, but I didn’t find any relevant references in a quick web search, so I thought I’d make a quick post with the solution.