Be sure procedural filterer is present before use

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/2407

src/js/contentscript.js

@@ -659,12 +659,15 @@ vAPI.DOMFilterer = class {
                     ...this.proceduralFilterer.selectors.values()
                 );
             }
+            const proceduralFilterer = this.proceduralFiltererInstance();
+            if ( proceduralFilterer !== null ) {
                 for ( const json of this.convertedProceduralFilters ) {
                     out.procedural.push(
-                    this.proceduralFiltererInstance().createProceduralFilter(json)
+                        proceduralFilterer.createProceduralFilter(json)
                     );
                 }
             }
+        }
         return out;
     }