Eliminate un-intuitive NPE (#11802)

* Eliminate un-intuitive NPE

Fixes #10947

Author: TatuLund

server/src/main/java/com/vaadin/ui/renderers/TextRenderer.java

@@ -52,9 +52,12 @@ public TextRenderer(String nullRepresentation) {
     public JsonValue encode(Object value) {
         if (value == null) {
             return super.encode(null);
-        } else {
-            return Json.create(value.toString());
         }
+        String stringValue = value.toString();
+        if (stringValue == null) {
+            return super.encode(null);
+        }
+        return Json.create(stringValue);
     }
 
     @Override