[BUG]: Extension list does not get updated correctly

[cnmarco]

Once table 'tx_t3monitoring_domain_model_extension' is filled, T3Monitor\T3monitoring\Service\Import\ExtensionImport::insertExtensionsInCustomTable() does not get called.
As far as I understand, this method only gets called when T3Monitor\T3monitoring\Service\Import\ExtensionImport::updateExtensionList() has returned true before. That method, however, can not return true, because it depends on a boolean return value of a core mehtod which in fact returns void.