diff --git a/TULIP/src/lu/list/itis/dkd/tui/space/SpatialPositioningManager.java b/TULIP/src/lu/list/itis/dkd/tui/space/SpatialPositioningManager.java
index d2c7e5c7750214dae0e4eda85f8cf9b2ccb02daf..f028777bef655a564f90d80cc7159dac24b31acf 100644
--- a/TULIP/src/lu/list/itis/dkd/tui/space/SpatialPositioningManager.java
+++ b/TULIP/src/lu/list/itis/dkd/tui/space/SpatialPositioningManager.java
@@ -260,13 +260,14 @@ public class SpatialPositioningManager implements SpatialEventListener {
             return null;
         }
 
-        for (TangibleObject object : spatialMatrix.getHorizontal()) {
-            BaseWidget widget = TangibleObjectManager.getWidget(object.getObjectId());
-            if (widget.getClass().equals(_class)) {
-                return (T) widget;
+        synchronized (this) {
+            for (TangibleObject object : new ArrayList<>(spatialMatrix.getHorizontal())) {
+                BaseWidget widget = TangibleObjectManager.getWidget(object.getObjectId());
+                if (widget.getClass().equals(_class)) {
+                    return (T) widget;
+                }
             }
         }
-
         return null;
     }