“Load subject, content and attachment from this email template” JS error

Clicking this button throws

ncaught TypeError: Cannot read property 'setContent' of null
    at Object.success (email-template-attachment-admin.js?ver=d06708e94f214c25ca58e211a9134360:53)
    at i (load-scripts.php?c=0&load[chunk_0]=jquery-core,utils,moxiejs,plupload&ver=d06708e94f214c25ca58e211a9134360:2)
    at Object.fireWith [as resolveWith] (load-scripts.php?c=0&load[chunk_0]=jquery-core,utils,moxiejs,plupload&ver=d06708e94f214c25ca58e211a9134360:2)
    at x (load-scripts.php?c=0&load[chunk_0]=jquery-core,utils,moxiejs,plupload&ver=d06708e94f214c25ca58e211a9134360:4)
    at XMLHttpRequest.c (load-scripts.php?c=0&load[chunk_0]=jquery-core,utils,moxiejs,plupload&ver=d06708e94f214c25ca58e211a9134360:4)

from this block in

	$( '#load_email_template' ).click( function(){
		if( $( '#email_template_selected' ).val() == '' )
			return;

		var data = {
			'action': 'acui_email_template_selected',
			'email_template_selected': $( '#email_template_selected' ).val(),
			'security': email_template_attachment_admin.security,
		};

		$.post( ajaxurl, data, function( response ) {
			var response = JSON.parse( response );
			$( '#title' ).val( response.title );
			tinyMCE.get( 'body_mail' ).setContent( response.content );
			$( '#email_template_attachment_id' ).val( response.attachment_id );
			if( response.attachment_url != '' ){
				$( '#email_template_attachment_file' ).val( response.attachment_url );
			}
			$( '#template_id' ).val( response.id );
			$( '#save_mail_template_options' ).click();
		});
	} );

specifically:

tinyMCE.get( ‘body_mail’ ).setContent( response.content );