Update AlertHtmlNodeRenderer.java

Author: rdestefa

commonmark-ext-gfm-alerts/src/main/java/org/commonmark/ext/gfm/alerts/internal/AlertHtmlNodeRenderer.java

@@ -24,7 +24,8 @@ public AlertHtmlNodeRenderer(HtmlNodeRendererContext context, Map<String, String
     @Override
     protected void renderAlert(Alert alert) {
         var type = alert.getType();
-        if (!allowedTypes.containsKey(type)) {
+        var defaultTitle = allowedTypes.get(type);
+        if (defaultTitle == null) {
             throw new IllegalStateException("Unknown alert type: " + type);
         }
         var cssClass = type.toLowerCase();
@@ -43,7 +44,7 @@ protected void renderAlert(Alert alert) {
         if (first instanceof AlertTitle) {
             renderChildren(first);
         } else {
-            htmlWriter.text(allowedTypes.get(type));
+            htmlWriter.text(defaultTitle);
         }
         htmlWriter.tag("/p");
         htmlWriter.line();