fix(engine): prevent NPE when terminating call activity

If a parent process has an active call activity element instance there is no guarantee that the called activity is still activate. When the called activity is completed it will try to complete the call activity element. At this point we try to apply the output mappings. If something goes wrong here an incident is created.
When we try to terminate the call activity at this point the called activity is already completed and no instance of it will be available. For this reason we must verify that this instance is not null before we try to terminate it.

engine/src/main/java/io/camunda/zeebe/engine/processing/processinstance/ProcessInstanceModificationProcessor.java

@@ -572,7 +572,9 @@ private void terminateElement(
     } else if (elementType == BpmnElementType.CALL_ACTIVITY) {
       final var calledActivityElementInstance =
           elementInstanceState.getInstance(elementInstance.getCalledChildInstanceKey());
-      terminateElement(calledActivityElementInstance, sideEffects);
+      if (calledActivityElementInstance != null && calledActivityElementInstance.canTerminate()) {
+        terminateElement(calledActivityElementInstance, sideEffects);
+      }
     }
 
     stateWriter.appendFollowUpEvent(

engine/src/test/java/io/camunda/zeebe/engine/processing/processinstance/ModifyProcessInstanceTest.java

@@ -17,6 +17,7 @@
 import io.camunda.zeebe.model.bpmn.builder.SubProcessBuilder;
 import io.camunda.zeebe.protocol.record.Record;
 import io.camunda.zeebe.protocol.record.RecordType;
+import io.camunda.zeebe.protocol.record.intent.IncidentIntent;
 import io.camunda.zeebe.protocol.record.intent.JobIntent;
 import io.camunda.zeebe.protocol.record.intent.ProcessInstanceIntent;
 import io.camunda.zeebe.protocol.record.intent.ProcessInstanceModificationIntent;
@@ -850,6 +851,54 @@ public void shouldActivateParallelGateway() {
         .hasSize(4);
   }
 
+  @Test
+  public void verifyCallActivityWithIncidentInOutputMappingCanBeTerminated() {
+    final var child = Bpmn.createExecutableProcess("child").startEvent().endEvent().done();
+    final var parent =
+        Bpmn.createExecutableProcess(PROCESS_ID)
+            .startEvent()
+            .callActivity("callActivity", c -> c.zeebeProcessId("child"))
+            .zeebeOutputExpression("x", "y")
+            .manualTask("task")
+            .endEvent()
+            .done();
+
+    ENGINE.deployment().withXmlResource(child).withXmlResource(parent).deploy();
+
+    final var processInstanceKey = ENGINE.processInstance().ofBpmnProcessId(PROCESS_ID).create();
+
+    final var callActivityElement =
+        RecordingExporter.processInstanceRecords(ProcessInstanceIntent.ELEMENT_ACTIVATED)
+            .withProcessInstanceKey(processInstanceKey)
+            .withElementId("callActivity")
+            .withElementType(BpmnElementType.CALL_ACTIVITY)
+            .getFirst();
+
+    Assertions.assertThat(
+            RecordingExporter.incidentRecords(IncidentIntent.CREATED)
+                .withProcessInstanceKey(processInstanceKey)
+                .getFirst())
+        .extracting(r -> r.getValue().getElementId())
+        .isEqualTo("callActivity");
+
+    ENGINE
+        .processInstance()
+        .withInstanceKey(processInstanceKey)
+        .modification()
+        .activateElement("task")
+        .terminateElement(callActivityElement.getKey())
+        .modify();
+
+    verifyThatRootElementIsActivated(processInstanceKey, "task", BpmnElementType.MANUAL_TASK);
+    verifyThatProcessInstanceIsCompleted(processInstanceKey);
+    Assertions.assertThat(
+            RecordingExporter.processInstanceRecords(ProcessInstanceIntent.ELEMENT_TERMINATED)
+                .withProcessInstanceKey(processInstanceKey)
+                .withElementId(callActivityElement.getValue().getElementId())
+                .exists())
+        .isTrue();
+  }
+
   private static void verifyThatRootElementIsActivated(
       final long processInstanceKey, final String elementId, final BpmnElementType elementType) {
     verifyThatElementIsActivated(processInstanceKey, elementId, elementType, processInstanceKey);