Just noticed, that with UI5 version 1.119.0 the getSecurityToken() function got replaced with securityTokenAvailable().
https://sapui5.hana.ondemand.com/#/api/sap.ui.model.odata.v2.ODataModel%23methods/getSecurityToken
// Returns the current security token if available; triggers a request to fetch the security token if it is not available.
const token = this.getModel().getSecurityToken() // Deprecated
// Returns a promise, which will resolve with the security token as soon as it is available.
const token = await this.getModel().securityTokenAvailable()