1-On a content app (category/contact/asset...)
2-Create a folder
3-Create a component under this folder
4-Remove the folder

Already have a fix for this.(ehe)

2013-06-13 10:23:46,013 INFO info.magnolia.ui.workbench.WorkbenchPresenter : Trying to re-sync workbench with no longer existing path e9d6b71d-1875-437f-af7c-25bc699f593f at workspace category. Will reset path to its configured root /.
2013-06-13 10:23:46,014 WARN info.magnolia.event.SimpleEventBus : Exception caught when dispatching event: null
java.lang.NullPointerException
at info.magnolia.ui.workbench.WorkbenchPresenter.select(WorkbenchPresenter.java:214)
at info.magnolia.ui.contentapp.browser.BrowserPresenter$1.onContentChanged(BrowserPresenter.java:163)
at info.magnolia.ui.api.event.ContentChangedEvent.dispatch(ContentChangedEvent.java:73)
at info.magnolia.ui.api.event.ContentChangedEvent.dispatch(ContentChangedEvent.java:1)
at info.magnolia.event.SimpleEventBus.fireEvent(SimpleEventBus.java:78)
at info.magnolia.event.ResettableEventBus.fireEvent(ResettableEventBus.java:70)
at info.magnolia.event.ResettableEventBus.fireEvent(ResettableEventBus.java:70)
at info.magnolia.ui.framework.action.ActivationAction.onPostExecute(ActivationAction.java:101)
at info.magnolia.ui.framework.action.AbstractCommandAction.execute(AbstractCommandAction.java:162)
at info.magnolia.ui.api.action.AbstractActionExecutor.execute(AbstractActionExecutor.java:75)
at info.magnolia.ui.contentapp.browser.BrowserPresenter.executeAction(BrowserPresenter.java:355)
at info.magnolia.ui.contentapp.browser.BrowserPresenter.onActionbarItemClicked(BrowserPresenter.java:318)
at info.magnolia.ui.actionbar.ActionbarPresenter.onActionbarItemClicked(ActionbarPresenter.java:190)