[Private Messaging] Problem with Private Messaging plugin

Hello today all of a sudden when I click on "Compose" in private messaging, doesen't load the pop-up window, even if I click on reply. I did not make any changes. I also tried to update the plugin, but it gives me the same error. How can I solve? In the error console it tells me:

perfect-scrollbar.min.js: 4 Uncaught TypeError: Can not read property 'length' of undefined

    at HTMLDivElement.i (perfect-scrollbar.min.js: 4)

    at HTMLDivElement.dispatch (jquery.js: 3)

    at HTMLDivElement.r.handle (jquery.js: 3)

Sorry for my english.