Fix: When footer is not enabled, avoid JS errors (#15365)

Author: dero

modules/infinite-scroll/infinity.js

@@ -202,6 +202,10 @@
 		var blog = document.getElementById( 'infinity-blog-title' );
 		var self = this;
 
+		if ( ! blog ) {
+			return;
+		}
+
 		blog.setAttribute( 'title', totop );
 		blog.addEventListener( 'click', function( e ) {
 			var sourceScroll = self.window.pageYOffset;
@@ -223,7 +227,12 @@
 			footerContainer,
 			width,
 			sourceBottom,
-			targetBottom;
+			targetBottom,
+			footerEnabled = this.footer && this.footer.el;
+
+		if ( ! footerEnabled ) {
+			return;
+		}
 
 		// Check if we have an id for the page wrapper
 		if ( 'string' === typeof this.footer.wrap ) {