magento2 – XML ​​declaration allowed only at the beginning of the document

I get this error in developer mode but works by default

Exception # 0 (Magento Framework Config Dom ValidationException): Themes layout update file
XML declaration allowed only at the beginning of the document
Line: 31
The first XML line is line 31 before its HTML comment



     
    
       
            
        
         
            
                
                 
                    addvideo_section
                    airhotels_uploadvideo_edit_tab_addvideo
                
                
            
        
    

Thank you