Add nullability annotation

platform/platform-api/src/com/intellij/openapi/ui/popup/util/BaseListPopupStep.java

@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2014 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package com.intellij.openapi.ui.popup.util;
 
 import com.intellij.openapi.ui.popup.ListPopupStep;
@@ -79,6 +65,7 @@ public class BaseListPopupStep<T> extends BaseStep<T> implements ListPopupStep<T
   }
 
   @Override
+  @Nullable
   public PopupStep onChosen(T selectedValue, final boolean finalChoice) {
     return FINAL_CHOICE;
   }