Command panel can be detached after preview without throwing an exception

spec/extensions/command-panel-spec.coffee

@@ -138,6 +138,8 @@ describe "CommandPanel", ->
           expect(previewItem.preview.text()).toBe "aaa bbb"
           expect(previewItem.preview.find(".match").text()).toBe "aaa"
 
+          rootView.trigger 'command-panel:toggle'
+
     describe "if the command is malformed", ->
       it "adds and removes an error class to the command panel and does not close it", ->
         rootView.trigger 'command-panel:toggle'

src/extensions/command-panel/command-panel.coffee

@@ -68,6 +68,7 @@ class CommandPanel extends View
     @rootView.focus()
     if @previewedOperations
       operation.destroy() for operation in @previewedOperations
+      @previewedOperations = undefined
     super
 
   execute: (command = @miniEditor.getText()) ->