failed to retrieve data for application tree users - umbraco

Question

I was attempting to add a 3rd party plug in Formulate to my umbraco site.
Something failed during the installation and I can no longer see any of the entries in the back office tree, although the site still runs.
When setting the config to debug:
(Umbraco version 7.3.4 assembly: 1.0.5820.25371)
Received an error from the server
Failed to retrieve data for application tree users
Value cannot be null.
Parameter name: value
EXCEPTION DETAILS:
System.ArgumentNullException: Value cannot be null.
Parameter name: value
STACKTRACE:
at Umbraco.Core.Cache.HttpRuntimeCacheProvider.GetCacheItem(String cacheKey, Func1 getCacheItem, Nullable1 timeout, Boolean isSliding, CacheItemPriority priority, CacheItemRemovedCallback removedCallback, CacheDependency dependency)
at Umbraco.Core.Cache.HttpRuntimeCacheProvider.GetCacheItem(String cacheKey, Func1 getCacheItem, Nullable1 timeout, Boolean isSliding, CacheItemPriority priority, CacheItemRemovedCallback removedCallback, String[] dependentFiles)
at Umbraco.Core.Services.ApplicationTreeService.GetAppTrees()
at Umbraco.Core.Services.ApplicationTreeService.GetApplicationTrees(String applicationAlias, Boolean onlyInitialized)
at Umbraco.Web.Trees.ApplicationTreeController.d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Threading.Tasks.TaskHelpersExtensions.d__3`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ApiControllerActionInvoker.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.ActionFilterAttribute.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Web.Http.Filters.ActionFilterAttribute.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.ActionFilterAttribute.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.ActionFilterAttribute.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Web.Http.Filters.ActionFilterAttribute.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.ActionFilterAttribute.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ActionFilterResult.d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.AuthorizationFilterAttribute.d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Dispatcher.HttpControllerDispatcher.d__1.MoveNext()

Answer 1

I heard from Nicholas Westby, the author of Formulate and that got my site back.
Hi James,
I'm guessing you are talking about Formulate (I have made several Umbraco packages). Here's the quick fix:
If you have source control, simply undo the changes made by Formulate. If not, here's a few more steps.
Remove the Formulate DLL's from the bin folder.
Remove Formulate references from the web.config.
Remove Formulate references from any of the files in the config folder (e.g., trees.config, applications.config, dashboard.config).
That should get your site working again. If you'd like some assistance getting Formulate up and running, post a message here and we can start troubleshooting: https://our.umbraco.org/projects/backoffice-extensions/formulate/formulate-questions/
Regards,
Nick