Add missing synchronization

Author: jisedlac

visualvm/core/src/org/graalvm/visualvm/core/ui/DataSourceViewProvider.java

@@ -123,7 +123,9 @@ void viewAdded(DataSourceView view) {
     }
 
     void viewRemoved(DataSourceView view) {
-        viewsCache.remove((X)view.getDataSource());
+        synchronized(viewsCache) {
+            viewsCache.remove((X)view.getDataSource());
+        }
     }
 
 }