Throw IllegalArgumentException when url is null in LinkHandlerFactory

extractor/src/main/java/org/schabi/newpipe/extractor/linkhandler/LinkHandlerFactory.java

@@ -43,6 +43,7 @@ public abstract class LinkHandlerFactory {
     ///////////////////////////////////
 
     public LinkHandler fromUrl(String url) throws ParsingException {
+        if (url == null) throw new IllegalArgumentException("url can not be null");
         final String baseUrl = Utils.getBaseUrl(url);
         return fromUrl(url, baseUrl);
     }