fix potential NPE in tearDown methods

Also, reset the link to static field after test run - to avoid holding heavy objects in memory till the end of all tests.

Author: asolntsev

java/test/org/openqa/selenium/ReferrerTest.java

@@ -114,9 +114,15 @@ public void startServers() {
 
   @AfterEach
   public void stopServers() {
-    safelyCall(() -> proxyServer.stop());
-    safelyCall(() -> server1.stop());
-    safelyCall(() -> server2.stop());
+    if (proxyServer != null) {
+      safelyCall(() -> proxyServer.stop());
+    }
+    if (server1 != null) {
+      safelyCall(() -> server1.stop());
+    }
+    if (server2 != null) {
+      safelyCall(() -> server2.stop());
+    }
   }
 
   private WebDriver createDriver(String pacUrl) {

java/test/org/openqa/selenium/devtools/CdpFacadeTest.java

@@ -56,6 +56,7 @@ public static void startServer() {
   @AfterAll
   public static void stopServer() {
     safelyCall(() -> server.stop());
+    server = null;
   }
 
   @Test

java/test/org/openqa/selenium/grid/router/RemoteWebDriverBiDiTest.java

@@ -96,8 +96,6 @@ void ensureBiDiSessionCreation() {
   @Ignore(IE)
   @Ignore(SAFARI)
   void canListenToLogs() throws ExecutionException, InterruptedException, TimeoutException {
-    driver = new Augmenter().augment(driver);
-
     try (LogInspector logInspector = new LogInspector(driver)) {
       CompletableFuture<ConsoleLogEntry> future = new CompletableFuture<>();
       logInspector.onConsoleEntry(future::complete);
@@ -135,11 +133,14 @@ void canNavigateToUrl() {
 
   @AfterEach
   void clean() {
-    driver.quit();
+    if (driver != null) {
+      driver.quit();
+    }
   }
 
   @AfterAll
   static void stopServer() {
     server.stop();
+    server = null;
   }
 }

java/test/org/openqa/selenium/grid/sessionmap/jdbc/JdbcBackedSessionMapTest.java

@@ -59,8 +59,14 @@ public static void createDB() throws SQLException {
 
   @AfterAll
   public static void killDBConnection() throws SQLException {
-    connection.close();
-    bus.close();
+    if (connection != null) {
+      connection.close();
+      connection = null;
+    }
+    if (bus != null) {
+      bus.close();
+      bus = null;
+    }
   }
 
   @Test

java/test/org/openqa/selenium/safari/CrossDomainTest.java

@@ -49,7 +49,11 @@ public static void startSecondServer() {
 
   @AfterAll
   public static void stopSecondServer() {
-    otherServer.stop();
+    if (otherServer != null) {
+      otherServer.stop();
+      otherServer = null;
+    }
+    otherPages = null;
   }
 
   @Test