我项目中报错Cannot read property 'getElementById' of undefined。不知道问题出在哪

调用这个函数就报错Cannot read property 'getElementById' of undefined

saveImg(){
					
	   var shareContent = document.getElementById("section");
					
		html2canvas(shareContent).then((canvas) => {
	         this.src1 = canvas.toDataURL() ;　　　　				
	    })

		setTimeout(()=>{
			this.selectSrc()
		},1500)
					　
	},