[platform] report assertion instead of failing with NPE if markCurrentCommandAsGlobal is called not inside command

This is a temporary workaround for KT-40787.

GitOrigin-RevId: f4b65d9df9bdd90ccf7ab449ce9cedd2c3adff0a

platform/platform-impl/src/com/intellij/openapi/command/impl/UndoManagerImpl.java

@@ -291,6 +291,10 @@ public class UndoManagerImpl extends UndoManager {
   }
 
   public void markCurrentCommandAsGlobal() {
+    if (myCurrentMerger == null) {
+      LOG.error("Must be called inside command");
+      return;
+    }
     myCurrentMerger.markAsGlobal();
   }