Prevent NPE if model assignment is removed while waiting to start

Author: davidkyle

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/action/TransportStartTrainedModelDeploymentAction.java

@@ -671,7 +671,11 @@ public boolean test(ClusterState clusterState) {
                 deploymentId
             ).orElse(null);
             if (trainedModelAssignment == null) {
-                // Something weird happened, it should NEVER be null...
+                // The assignment may be null if it was stopped by another action while waiting
+                this.exception = new ElasticsearchStatusException(
+                    "Error waiting for the model deployment to start. The trained model assignment was removed while waiting",
+                    RestStatus.BAD_REQUEST
+                );
                 logger.trace(() -> format("[%s] assignment was null while waiting for state [%s]", deploymentId, waitForState));
                 return true;
             }