download : dialog box failed to open (as2)

• You did not pass a value for the url parameter.
• The parameters passed are of the incorrect type or format.
• The url parameter has a length of 0.
• A security violation occurred; your SWF file attempted to access a file from a server that is outside your SWF file's security sandbox.
• Another browse session is already in progress. A browse session can be started by FileReference.browse(), FileReferenceList.browse(), or FileReference.download().
• The protocol is not HTTP or HTTPS.