title: sap.ui.getCore().byId() returns no element
body: <p>I created an UI5 app and now want to use <code>sap.ui.getCore().byId(id)</code> to access existing controls. In the same controller/view I can access them via <code>this.byId(id)</code>, but to access controls in other files (e.g. controllers) I need <code>sap.ui.getCore().byId(id)</code>.</p>
  
  <p>However, while <code>sap.ui.getCore()</code> returns a core object, I cannot access controls via the <code>byId()</code> function of it, I get <code>undefined</code> instead. I already built an UI5 app and there I have no issue with this function call.</p>
  
  <p>Is there something I have to configure for this to work?</p>