message: start transaction - error code must not be null
diff: @@ -1412,14 +1412,15 @@ export default class StripeBillingIntegration extends BillingIntegration {
  }
  public async precheckStartTransactionPrerequisites(user: User): Promise<StartTransactionErrorCode[]> {
  + const errorCodes: StartTransactionErrorCode[] = [];
  // Check billing prerequisites
  if (!this.settings.billing.isTransactionBillingActivated) {
  // Nothing to check - billing of transactions is not yet ON
  - return null;
  + return errorCodes;
  }
  if (this.isUserInternal(user)) {
  // Nothing to check - we do not bill internal user's transactions
  - return null;
  + return errorCodes;
  }
  // Make sure the STRIPE connection is ok
  try {
  @@ -1435,7 +1436,6 @@ export default class StripeBillingIntegration extends BillingIntegration {
  return [StartTransactionErrorCode.BILLING_NO_SETTINGS];
  }
  // Check all settings that are necessary to bill a transaction
  - const errorCodes: StartTransactionErrorCode[] = [];
  try {
  await this.checkTaxPrerequisites(); // Checks that the taxID is still valid
  } catch (error) {
  