Dynamics 365 Can't Edit or Create workflow

Hi,

I updated Dynamics CRM 2011 to Dynamics 365 but I'm getting an error whenever I try to edit a workflow or create a new one. Note that, this only happens when it's in Draft. When it's Activated I can open the workflow and see all the steps.

However, as soon as it's deactivated, I get the following error:

[CrmException: Exception retrieving custom activity info - Object reference not set to an instance of an object.]
   at Microsoft.Crm.Workflow.ObjectModel.CustomActivityInfoMetadataBase.GetCustomActivityInfo(ActivityInfo activityInfo, Int32 isolationMode)
   at Microsoft.Crm.Workflow.ObjectModel.CustomActivityInfoMetadataBase.GetCustomActivityInfo(Guid pluginTypeId)
   at Microsoft.Crm.Web.Sfa.WorkflowDetailPage.ConfigureCustomActivityStepMenu(CommandBarPopup addStepButton)
   at Microsoft.Crm.Web.Sfa.WorkflowDetailPage.BuildStepMenu()
   at Microsoft.Crm.Web.Sfa.WorkflowDetailPage.ConfigureWorkflowStepMenu()
   at Microsoft.Crm.Web.Sfa.WorkflowDetailPage.ConfigureMenus()
   at Microsoft.Crm.Application.Controls.PageManager.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

[HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown.]
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)