Hi Jonas,
I am not sure about what you are trying to do in this onAfterRendering method, but the getProperty() method of the odata model, need three input parameters (Path, Context, and IncludeExpandEntries). So to access the value from first entity of the model, your code should be,
var oModel = sap.ui.getCore().getModel("model1");
var value = oModel.getProperty("Property", new sap.ui.model.Context(oModel, "/aSet/0"), false));
Regards,
Vishweshwara P.K.M.