Hi,
because your xml fragment is not embedded into your xml view but is a dialog and you are also not setting the fragment ID of your fragment your coding will not work.
So either you use sap.ui.getCore().byId("ApprovalInput"); to get your control, or you set an ID for your fragment (this._oViewSearchDialog = sap.ui.xmlfragment("frgSearchHelpDialog", "Demo.view.SearchHelpDialog",this);) and determine the control via sap.ui.core.Fragment.byId("frgSearchHelpDialog", "ApprovalInput");
Regards,
Florian