Fixed drag and drop failure when message dragged from email client (#20451)

When dragging message form email client on Windows, item.webkitGetAsEntry()
might return null creating NPE on the client side. Added additional checks
for this situation.

Change-Id: I569f7e6d0d7b137f24be53d1fbce384695ae8c73

client/src/main/java/com/vaadin/client/ui/dd/VHtml5DragEvent.java

@@ -90,8 +90,11 @@ public class VHtml5DragEvent extends NativeEvent {
         // Chrome >= v21 and Opera >= v?
         if (this.dataTransfer.items) {
             var item = this.dataTransfer.items[fileIndex];
-            if (item.webkitGetAsEntry) {
-                return item.webkitGetAsEntry().isFile;
+            if (typeof item.webkitGetAsEntry == "function") {
+                var entry = item.webkitGetAsEntry();
+                if (typeof entry !== "undefined" && entry !== null) {
+                    return entry.isFile;
+                }
             }
         }