Better error handling trying to enable HTML5 DnD for mobile from thread (#12170)

- Informative error message
- Reset back to disabled state when enabling fails
- Incorrect usage also detectable using non-mobile devices

Fixes #12152

1 files changed, 7 insertions, 0 deletions

diff --git a/server/src/main/java/com/vaadin/ui/UI.java b/server/src/main/java/com/vaadin/ui/UI.java
index 5891addca4..7430c04bb9 100644
--- a/server/src/main/java/com/vaadin/ui/UI.java
+++ b/server/src/main/java/com/vaadin/ui/UI.java
@@ -1987,6 +1987,13 @@ public abstract class UI extends AbstractSingleComponentContainer
             getState().enableMobileHTML5DnD = enabled;
 
             if (isMobileHtml5DndEnabled()) {
+                if (VaadinService.getCurrentRequest() == null) {
+                    getState().enableMobileHTML5DnD = false;
+                    throw new IllegalStateException("HTML5 DnD cannot be "
+                            + "enabled for mobile devices when current "
+                            + "VaadinRequest cannot be accessed. Call this "
+                            + "method from init(VaadinRequest) to ensure access.");
+                }
                 loadMobileHtml5DndPolyfill();
             }
         }