YARN-11597. Fix NPE when loading static files in SLSWebApp

Author: zuston

hadoop-tools/hadoop-sls/src/main/java/org/apache/hadoop/yarn/sls/web/SLSWebApp.java

@@ -136,6 +136,7 @@ public void start() throws Exception {
     String webRootDir = getClass().getClassLoader().getResource("html").
         toExternalForm();
     staticHandler.setResourceBase(webRootDir);
+    staticHandler.start();
 
     Handler handler = new AbstractHandler() {
       @Override