fix: Catch unexpected null pointer exception (#70)

### Summary

Catch unexpected null pointer exception and log error
retry on null response

### Checklist
* [x]Does your PR title have the correct title format?
* Does your PR have a breaking change?: no

gradle.properties

@@ -1,4 +1,4 @@
-ARTIFACT_VERSION=1.9.0
+ARTIFACT_VERSION=1.9.1
 ARTIFACT_GROUP=com.amplitude
 POM_PACKAGING=jar
 POM_DESCRIPTION=Amplitude Java SDK

src/main/java/com/amplitude/Constants.java

@@ -6,7 +6,7 @@ public interface Constants {
 
   int NETWORK_TIMEOUT_MILLIS = 10000;
   String SDK_LIBRARY = "amplitude-java";
-  String SDK_VERSION = "1.9.0";
+  String SDK_VERSION = "1.9.1";
 
   int MAX_PROPERTY_KEYS = 1024;
   int MAX_STRING_LENGTH = 1000;

src/main/java/com/amplitude/HttpTransport.java

@@ -54,6 +54,10 @@ public void sendEventsWithRetry(List<Event> events) {
     sendEvents(events)
         .thenAcceptAsync(
             response -> {
+              if (response == null) {
+                logger.debug("Unexpected null response", "Retry events.");
+                retryEvents(events, new Response());
+              }
               Status status = response.status;
               if (shouldRetryForStatus(status)) {
                 retryEvents(events, response);
@@ -104,6 +108,8 @@ private CompletableFuture<Response> sendEvents(List<Event> events) {
             logger.debug("SEND", events, response);
           } catch (AmplitudeInvalidAPIKeyException e) {
             throw new CompletionException(e);
+          } catch (Exception e) {
+            logger.error("Unexpected exception", Utils.getStackTrace(e));
           }
           return response;
         });

src/main/java/com/amplitude/Utils.java

@@ -3,6 +3,9 @@
 import org.json.JSONArray;
 import org.json.JSONObject;
 
+import java.io.PrintWriter;
+import java.io.StringWriter;
+
 public class Utils {
   public static String getStringValueWithKey(JSONObject json, String key) {
     return json.has(key) && json.getString(key) != null ? json.getString(key) : "";
@@ -32,4 +35,11 @@ public static int[] convertJSONArrayToIntArray(JSONObject json, String key) {
   public static boolean isEmptyString(String s) {
     return (s == null || s.length() == 0);
   }
+
+  public static String getStackTrace(Exception e) {
+    StringWriter sw = new StringWriter();
+    PrintWriter pw = new PrintWriter(sw);
+    e.printStackTrace(pw);
+    return sw.toString();
+  }
 }

src/test/java/com/amplitude/util/MockURLStreamHandler.java

@@ -5,6 +5,7 @@
 import java.util.HashMap;
 import java.util.Map;
 
+import static org.mockito.Mockito.lenient;
 import static org.mockito.Mockito.when;
 
 public class MockURLStreamHandler extends URLStreamHandler implements URLStreamHandlerFactory {
@@ -26,9 +27,9 @@ protected URLConnection openConnection(URL url) throws IOException {
   protected URLConnection openConnection(URL u, Proxy p) throws IOException {
     URLConnection connection = connections.get(u);
     if (p.equals(Proxy.NO_PROXY)) {
-      when(((HttpURLConnection) connection).usingProxy()).thenReturn(false);
+      lenient().when(((HttpURLConnection) connection).usingProxy()).thenReturn(false);
     } else {
-      when(((HttpURLConnection) connection).usingProxy()).thenReturn(true);
+      lenient().when(((HttpURLConnection) connection).usingProxy()).thenReturn(true);
     }
     return connection;
   }