uniapp createSelectorQuery(). Select get returns null error

Error message ：
An SelectorQuery call is ignored because no proper page or component is found. Please considering using SelectorQuery.in to specify a proper one.

terms of settlement ：

/*#ifdef APP-PLUS*/// app
				 let info= uni.createSelectorQuery().in(this).select('#tableHead');
				 info.boundingClientRect(data => {
    
				   // data Is the height of the current element 
				   this.headerHeight = data.height;
				 }).exec();
				/*#endif*/
				/*#ifdef MP*/// Applet 
				const query = this.createSelectorQuery().select('#tableHead');
				query.fields({
    
				  size: true
				}, data => {
    
				 this.headerHeight = data.height;
				}).exec();
				/* #endif*/