fix code

Author: seawinde

fe/fe-core/src/main/java/org/apache/doris/nereids/rules/exploration/mv/AbstractMaterializedViewWindowRule.java

@@ -127,6 +127,9 @@ protected Plan rewriteQueryByView(MatchMode matchMode, StructInfo queryStructInf
 
     // Check the tempRewrittenPlan is valid, should only contain logical project, scan or filter
     protected boolean checkTmpRewrittenPlanIsValid(Plan tempRewrittenPlan) {
+        if (tempRewrittenPlan == null) {
+            return false;
+        }
         return tempRewrittenPlan.accept(new DefaultPlanVisitor<Boolean, Void>() {
             @Override
             public Boolean visit(Plan plan, Void context) {