Add resume listener for immediate installs, no-op loadBundle if currentActivity is null

Author: geof90

android/app/src/main/java/com/microsoft/codepush/react/CodePushNativeModule.java

@@ -102,7 +102,11 @@ private void loadBundle() {
         mCodePush.clearDebugCacheIfNeeded();
         final Activity currentActivity = getCurrentActivity();
 
-        if (!ReactActivity.class.isInstance(currentActivity)) {
+        if (currentActivity == null) {
+            // The currentActivity can be null if it is backgrounded / destroyed, so we simply
+            // no-op to prevent any null pointer exceptions.
+            return;
+        } else if (!ReactActivity.class.isInstance(currentActivity)) {
             // Our preferred reload logic relies on the user's Activity inheriting
             // from the core ReactActivity class, so if it doesn't, we fallback
             // early to our legacy behavior.
@@ -379,7 +383,12 @@ protected Void doInBackground(Void... params) {
                     mSettingsManager.savePendingUpdate(pendingHash, /* isLoading */false);
                 }
 
-                if (installMode == CodePushInstallMode.ON_NEXT_RESUME.getValue()) {
+                if (installMode == CodePushInstallMode.ON_NEXT_RESUME.getValue() ||
+                    // We also add the resume listener if the installMode is IMMEDIATE, because
+                    // if the current activity is backgrounded, we want to reload the bundle when
+                    // it comes back into the foreground.
+                    installMode == CodePushInstallMode.IMMEDIATE.getValue()) {
+                    
                     // Store the minimum duration on the native module as an instance
                     // variable instead of relying on a closure below, so that any
                     // subsequent resume-based installs could override it.