Calling methods which use cluster under the hood can cause NullPointerExecption

[yuvalshi0]

Versions used

Akka version: latest (code on master)

Description

Consider the following code:

abstract class SpecBase(kafkaPort: Int) extends ScalatestKafkaSpec(kafkaPort) {
  protected def this() = this(kafkaPort = -1)
}

class FooTest extends SpecBase with TestcontainersKafkaLike {
  val bar = brokerContainers.head.getBrokerNum

}

Running this suite will result in the following exception:

An exception or error caused a run to abort: Cannot invoke "akka.kafka.testkit.internal.KafkaContainerCluster.getBrokers()" because the return value of "akka.kafka.testkit.internal.TestcontainersKafka$Spec.akka$kafka$testkit$internal$TestcontainersKafka$Spec$$cluster()" is null 
...

Of course, the issue here is that I'm trying to get information regarding the cluster when before I even started it.

Expected Behavior

Please describe the expected behavior of the issue, starting from the first action.
A proper exception, saying the cluster hasn't been started, will be thrown.

Actual Behavior

The method returns a NullPointerExecption

Suggested Solution

Check by default if the cluster has been started, like other methods do: