[Bug]: NPE on ApiException.getMessage()

[luis901101]

What happened?

When an ApiException is thrown and exception.printStackTrace() or just exception.getMessage() is called, then a NullPointerException is thrown due to this line in the getMessage() function implementation:

public String getMessage() {
        return String.format("Message: %s%nHTTP response code: %s%nHTTP response body: %s%nHTTP response headers: %s", super.getMessage(), this.getCode(), this.getResponseBody(), this.getResponseHeaders().toString());
    }

If this.getResponseHeaders() is null, then .toString() throws NPE.

Steps to reproduce?

When sending a notification fails:

apiClient.createNotificationAsync(
                notification,
                new ApiCallback<>() {
                    @Override
                    public void onFailure(ApiException e, int statusCode, Map<String, List<String>> responseHeaders) {
                        logger.info("Notification send failed with status code: "+ statusCode);
                        if(e != null) e.printStackTrace();
                    }

                    @Override
                    public void onSuccess(CreateNotificationSuccessResponse result, int statusCode, Map<String, List<String>> responseHeaders) {
                        System.out.println("Push notification sent: " + result.toJson());
                    }

                    @Override
                    public void onUploadProgress(long bytesWritten, long contentLength, boolean done) {

                    }

                    @Override
                    public void onDownloadProgress(long bytesRead, long contentLength, boolean done) {

                    }
                }
            );

What did you expect to happen?

exception.printStackTrace() or exception.getMessage() should not throw NPE.

Relevant log output

Exception in thread "OkHttp Dispatcher" java.lang.NullPointerException: Cannot invoke "java.util.Map.toString()" because the return value of "com.onesignal.client.ApiException.getResponseHeaders()" is null
	at com.onesignal.client.ApiException.getMessage(ApiException.java:164)
	at java.base/java.lang.Throwable.getLocalizedMessage(Throwable.java:397)
	at java.base/java.lang.Throwable.toString(Throwable.java:496)
	at java.base/java.lang.String.valueOf(String.java:4218)
	at java.base/java.io.PrintStream.println(PrintStream.java:1047)
	at java.base/java.lang.Throwable$WrappedPrintStream.println(Throwable.java:763)
	at java.base/java.lang.Throwable.printStackTrace(Throwable.java:670)
	at java.base/java.lang.Throwable.printStackTrace(Throwable.java:659)
	at java.base/java.lang.Throwable.printStackTrace(Throwable.java:650)
	at com.myapp.backend.notification.client.NotificationClient$2.onFailure(NotificationClient.java:97)
	at com.onesignal.client.ApiClient$1.onResponse(ApiClient.java:1038)
	at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:833)

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[carlos-contreras14]

bump