Error "Cannot read property 'c' of null" when open a human task in project designer

Error log :
09:55:36,323 ERROR [org.kie.workbench.common.services.backend.logger.GenericErrorLoggerServiceImpl] (default task-11) Error from user: wbadmin Error ID: 804263183 Location: LibraryPerspective|$ProjectScreen,jbpm.designer?path_uri=default://master@MySpace/IT_Orders/src/main/resources/org/jbpm/demo/itorders/orderhardware.bpmn2&file_name=orderhardware.bpmn2&has_version_support=true,BPMNDiagramEditor?path_uri=default://master@MySpace/IT_Orders/src/main/resources/org/jbpm/demo/itorders/place-order.bpmn&file_name=place-order.bpmn&has_version_support=true[EDiagramEditorPropertiesScreen,] Exception: Uncaught exception: Client-side exception occurred although RPC call succeeded. Caused by: (TypeError) : Cannot read property 'c' of null