fix possible NPE and add test

Author: Pezzah

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## 3.4.2 (2018-11-28)
+
+* Ensure session counts are thread safe
+  [#122](https://github.com/bugsnag/bugsnag-java/pull/122)
+
 ## 3.4.1
 
 (Skipped, duplicate of 3.4.0)

bugsnag/src/main/java/com/bugsnag/Report.java

@@ -137,16 +137,16 @@ Map<String, Object> getSession() {
     void setSession(Session session) {
         if (session == null) {
             sessionMap = null;
+        } else {
+            sessionMap = new HashMap<String, Object>();
+            sessionMap.put("id", session.getId());
+            sessionMap.put("startedAt", session.getStartedAt());
+
+            Map<String, Object> handledCounts = new HashMap<String, Object>();
+            handledCounts.put("handled", session.getHandledCount());
+            handledCounts.put("unhandled", session.getUnhandledCount());
+            sessionMap.put("events", handledCounts);
         }
-
-        sessionMap = new HashMap<String, Object>();
-        sessionMap.put("id", session.getId());
-        sessionMap.put("startedAt", session.getStartedAt());
-
-        Map<String, Object> handledCounts = new HashMap<String, Object>();
-        handledCounts.put("handled", session.getHandledCount());
-        handledCounts.put("unhandled", session.getUnhandledCount());
-        sessionMap.put("events", handledCounts);
     }
 
     /**

bugsnag/src/test/java/com/bugsnag/NotificationTest.java

@@ -5,6 +5,7 @@
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.junit.Before;
@@ -31,6 +32,9 @@ public void setUp() {
         config.appVersion = "1.2.3";
         config.releaseStage = "dev";
         report = new Report(config, new RuntimeException());
+
+        // Only include properties with non-null values
+        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
     }
 
     private JsonNode generateJson(ObjectMapper mapper,
@@ -67,6 +71,17 @@ public void testWithoutSessionSerialisation() throws Throwable {
         assertNull(rootNode.get("events").get("session"));
     }
 
+    @Test
+    public void testNullSession() throws Throwable {
+        report.setSession(null);
+
+        JsonNode rootNode = generateJson(mapper, config, report);
+        validateErrorReport(rootNode);
+
+        JsonNode session = rootNode.get("events").get(0).get("session");
+        assertNull(session);
+    }
+
     private void validateErrorReport(JsonNode rootNode) {
         assertNotNull(rootNode);
         assertNotNull(rootNode.get("apiKey").asText());