NPE when requesting incident statistics on startup

Steps

• Operate is just started with demo data generation
• The dashboard is being opened

Current behaviour:

• I see error on frontend, telling that "Incidents by Error Message" can't be loaded
• I see exception in logs:

  java.lang.NullPointerException: null
          at org.camunda.operate.es.reader.IncidentStatisticsReader.lambda$null$0(IncidentStatisticsReader.java:238) ~[classes!/:?]
          at java.util.ArrayList.forEach(ArrayList.java:1257) ~[?:1.8.0_191]
          at org.camunda.operate.es.reader.IncidentStatisticsReader.lambda$getIncidentStatisticsByError$1(IncidentStatisticsReader.java:234) ~[classes!/:?]
          at java.util.ArrayList.forEach(ArrayList.java:1257) ~[?:1.8.0_191]
          at org.camunda.operate.es.reader.IncidentStatisticsReader.getIncidentStatisticsByError(IncidentStatisticsReader.java:229) ~[classes!/:?]
          at org.camunda.operate.rest.IncidentRestService.getIncidentStatisticsByError(IncidentRestService.java:44) ~[classes!/:?]
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_191]
          at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_191]
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_191]
          at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_191]
  

Expected:

• we should have null checks for such cases
• no exception and some response is available (we can replace workflow names with bpmnProcessId)