Fixing NPE when ItemCaptionGenerator returns null (#11435)

* Fixing NPE when ItemCaptionGenerator returns null

Adding similar logic in setDataProvider(...) as used elsewhere in ComboBox.java (e.g. generateData(..) see line 314)

fixes issue #11434

server/src/main/java/com/vaadin/ui/ComboBox.java

@@ -453,9 +453,18 @@ public class ComboBox<T> extends AbstractSingleSelect<T>
         // Must do getItemCaptionGenerator() for each operation since it might
         // not be the same as when this method was invoked
         setDataProvider(listDataProvider, filterText -> item -> captionFilter
-                .test(getItemCaptionGenerator().apply(item), filterText));
+                .test(getItemCaptionOfItem(item), filterText));
     }
 
+    // Helper method for the above to make lambda more readable            
+    private String getItemCaptionOfItem(T item) {
+        String caption = getItemCaptionGenerator().apply(item);
+        if (caption == null) {
+            caption = "";
+        }    
+        return caption;
+    }
+                
     /**
      * Sets the data items of this listing and a simple string filter with which
      * the item string and the text the user has input are compared.