由getPackageManager（）引起的空指针异常

Question

因此，我尝试在this documentation的帮助下实施gcm或我的应用。但是，在尝试启动我的应用程序时，我收到以下错误：

 java.lang.RuntimeException: An error occured while executing doInBackground()
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference

以下是相关代码：

private static int getAppVersion(Context context) {
        try {
            PackageInfo packageInfo = context.getPackageManager()
                    .getPackageInfo(context.getPackageName(), 0);
            return packageInfo.versionCode;
        } catch (PackageManager.NameNotFoundException e) {
            // should never happen
            throw new RuntimeException("Could not get package name: " + e);
        }
    }

我的错误是什么？

编辑：我在哪里使用＆＃34; getAppVersion＆＃34;：

private String getRegistrationId(Context context) {
        final SharedPreferences prefs = getGCMPreferences(context);
        String registrationId = prefs.getString(PROPERTY_REG_ID, "");
        if (registrationId.isEmpty()) {
            Log.i(TAGD, "Registration not found.");
            return "";
        }
        // Check if app was updated; if so, it must clear the registration ID
        // since the existing registration ID is not guaranteed to work with
        // the new app version.
        int registeredVersion = prefs.getInt(PROPERTY_APP_VERSION, Integer.MIN_VALUE);
        int currentVersion = getAppVersion(context);
        if (registeredVersion != currentVersion) {
            Log.i(TAGD, "App version changed.");
            return "";
        }
        return registrationId;
    }

和

private void storeRegistrationId(Context context, String regId) {
        final SharedPreferences prefs = getGCMPreferences(context);
        int appVersion = getAppVersion(context);
        Log.i(TAGD, "Saving regId on app version " + appVersion);
        SharedPreferences.Editor editor = prefs.edit();
        editor.putString(PROPERTY_REG_ID, regId);
        editor.putInt(PROPERTY_APP_VERSION, appVersion);
        editor.apply();
    }

我从哪里开始getRegistrationId： 在onCreate方法中，

if (checkPlayServices()) {
            gcm = GoogleCloudMessaging.getInstance(this);
            regid = getRegistrationId(context);

            if (regid.isEmpty()) {
                registerInBackground();
            }
        } else {
            Log.v(TAGD, "No valid GPS APK found.");
        }