mdPanel: Can't re-open a closed panel

[chrisguerrero]

Actual Behavior:

• What is the issue? * The following error is thrown when attempting to call the open() method on an MdPanelRef that has already been created, opened, and then closed:

TypeError: Cannot read property 'push' of null

• What is the expected behavior?

CodePen (or steps to reproduce the issue): *

• CodePen Demo which shows your issue: http://codepen.io/chrisguerrero/pen/zBZWyx
• Details: I've modified the panel demo from the docs to call the $mdPanel.create() method the first time the panel is shown, then store a reference to the MdPanelRef. The second time you try to open the modal, the show() method is called on the MdPanelRef, which throws the error.

Angular Versions: *

• Angular Version: 1.5.5
• Angular Material Version: v1.1.0-rc.5-master-15da974

Additional Information:

• Browser Type: * All
• Browser Version: * All
• OS: * All
• Stack Traces:

TypeError: Cannot read property 'push' of null
    at MdPanelRef._configureScrollListener (VM3709 angular-material.js:14477)
    at MdPanelRef._addEventListeners (VM3709 angular-material.js:14368)
    at addListeners (VM3709 angular-material.js:13989)
    at processQueue (VM3702 angular.js:15961)
    at VM3702 angular.js:15977
    at Scope.$eval (VM3702 angular.js:17229)
    at Scope.$digest (VM3702 angular.js:17045)
    at Scope.$apply (VM3702 angular.js:17337)
    at HTMLButtonElement.<anonymous> (VM3702 angular.js:25023)
    at defaultHandlerWrapper (VM3702 angular.js:3456)

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[devversion]

@chrisguerrero Thanks for the awesome issue and related PR.