Fix the bug that stopping CommandCenter may throw NPE when it was not initialized (#2630)

sentinel-transport/sentinel-transport-simple-http/src/main/java/com/alibaba/csp/sentinel/transport/command/SimpleHttpCommandCenter.java

@@ -158,7 +158,10 @@ public class SimpleHttpCommandCenter implements CommandCenter {
                 CommandCenterLog.warn("Error when releasing the server socket", e);
             }
         }
+
+        if (bizExecutor != null) {
             bizExecutor.shutdownNow();
+        }
         executor.shutdownNow();
         TransportConfig.setRuntimePort(PORT_UNINITIALIZED);
         handlerMap.clear();

sentinel-transport/sentinel-transport-simple-http/src/test/java/com/alibaba/csp/sentinel/transport/command/http/CommandCenterTest.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 1999-2018 Alibaba Group Holding Ltd.
+ *
+ * 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.
+ */
+package com.alibaba.csp.sentinel.transport.command.http;
+
+import com.alibaba.csp.sentinel.command.CommandCenterProvider;
+import com.alibaba.csp.sentinel.transport.CommandCenter;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class CommandCenterTest {
+    @Test
+    public void stopCommandCenter(){
+        CommandCenter commandCenter =  CommandCenterProvider.getCommandCenter();
+        try {
+            commandCenter.stop();
+        } catch (Exception e) {
+            Assert.fail();
+        }
+    }
+    @Test
+    public void startCommandCenter(){
+        CommandCenter commandCenter =  CommandCenterProvider.getCommandCenter();
+        try {
+            commandCenter.start();
+        } catch (Exception e) {
+            Assert.fail();
+        }
+    }
+    @Test
+    public void beforeStartCommandCenter(){
+        CommandCenter commandCenter =  CommandCenterProvider.getCommandCenter();
+        try {
+            commandCenter.beforeStart();
+        } catch (Exception e) {
+            Assert.fail();
+        }
+    }
+}