Fix adding document to clipboard

Steps to reproduce:

• Navigate to a folder containing several documents.
• Select all documents in the folder.
• Click on the Add to Clipboard multiple-select document action from the strip at the top of the page.
• Navigate to another workspace/folder.
• Create a new folder.
• Click on the clipboard from the left menu
• Click on the COPY button to copy the items from the clipboard to the new folder.
• Click on one of the documents.
• Click on the Add to Clipboard document action.
  => Document not added to clipboard.
  => Error appears in console:

  Uncaught TypeError: Cannot read property 'contains' of undefined
      at HTMLElement.toggle (elements.html:48515)
      at HTMLElement.handler (nuxeo-app.html:2283)
      at Object._fire (nuxeo-app.html:7171)
      at Object.forward (nuxeo-app.html:7566)
      at Object.click (nuxeo-app.html:7529)
      at HTMLElement._handleNative (nuxeo-app.html:6949)
  toggle	@	elements.html:48515
  handler	@	nuxeo-app.html:2283
  _fire	@	nuxeo-app.html:7171
  forward	@	nuxeo-app.html:7566
  click	@	nuxeo-app.html:7529
  _handleNative	@	nuxeo-app.html:6949