[FindBugs] Resolve NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE

Possible null pointer dereference in com.vexus2.jenkins.chatwork.jenkinschatworkplugin.ChatworkPublisher.createExtraVariables() due to return value of called method

src/main/java/com/vexus2/jenkins/chatwork/jenkinschatworkplugin/ChatworkPublisher.java

@@ -3,6 +3,7 @@
 import com.vexus2.jenkins.chatwork.jenkinschatworkplugin.api.ChatworkClient;
 import com.vexus2.jenkins.chatwork.jenkinschatworkplugin.api.Room;
 import com.vexus2.jenkins.chatwork.jenkinschatworkplugin.api.RoomComparator;
+import edu.umd.cs.findbugs.annotations.NonNull;
 import hudson.Extension;
 import hudson.Launcher;
 import hudson.model.AbstractBuild;
@@ -50,6 +51,7 @@ public class ChatworkPublisher extends Publisher {
   private final Boolean notifyOnNotBuilt;
   private final Boolean notifyOnAborted;
 
+  @NonNull
   private transient AbstractBuild build;
   private transient BuildListener listener;
 
@@ -232,11 +234,16 @@ private Map<String, String> createExtraVariables() {
       variables.put("PAYLOAD_SUMMARY", analyzePayload(payloadJson));
     }
 
-    variables.put("BUILD_RESULT", build.getResult().toString());
+    variables.put("BUILD_RESULT", getBuildResult().toString());
 
     return variables;
   }
 
+  @NonNull
+  private Result getBuildResult() {
+    return build.getResult();
+  }
+
   private static String analyzePayload(String payloadJson) {
     JSONObject json;
     try{