Check that variable isnt null when initializing ds reference

public/app/features/dashboard-scene/serialization/DashboardSceneSerializer.ts

@@ -345,6 +345,7 @@ export class V2DashboardSerializer
     // initialize autossigned variable ds references map
     if (saveModel?.variables) {
       for (const variable of saveModel.variables) {
+        if (variable) {
           // for query variables that dont have a ds defined add them to the list
           if (variable.kind === 'QueryVariable' && !variable.spec.query.datasource?.name) {
             const datasourceType = variable.spec.query.group || undefined;
@@ -352,6 +353,7 @@ export class V2DashboardSerializer
           }
         }
       }
+    }
 
     // initialize annotations ds references map
     if (saveModel?.annotations) {