App crashes due to network issue

[chihuazhang]

Studies have shown that mobile apps are prone to crashes due to a network issue, mainly because successful network calls return unexpected or no data. And we found that when this app (version 2.1)'s network returned data is null or some fields are null (JSON format), the app has crashed. the specific crash information is as follows:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toLowerCase(java.util.Locale)' on a null object reference
at com.afollestad.polar.util.WallpaperUtils.download(WallpaperUtils.java:337)
at com.afollestad.polar.viewer.ViewerPageFragment.onOptionsItemSelected(ViewerPageFragment.java:81)
at com.afollestad.polar.viewer.ViewerActivity.onOptionsItemSelected(ViewerActivity.java:234)
at android.app.Activity.onMenuItemSelected(Activity.java:2908)
at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:408)
at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:198)
at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:113)
at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:113)
at android.support.v7.app.ToolbarActionBar$2.onMenuItemClick(ToolbarActionBar.java:69)
at android.support.v7.widget.Toolbar$1.onMenuItemClick(Toolbar.java:206)
at android.support.v7.widget.ActionMenuView$MenuBuilderCallback.onMenuItemSelected(ActionMenuView.java:776)
at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:156)
at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:969)
at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:959)
at android.support.v7.widget.ActionMenuView.invokeItem(ActionMenuView.java:623)
at android.support.v7.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:154)
at android.view.View.performClick(View.java:5198)
at android.view.View$PerformClick.run(View.java:21147)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(int)' on a null object reference
at com.afollestad.polar.fragments.WallpapersFragment$4.onRetrievedWallpapers(WallpapersFragment.java:243)
at com.afollestad.polar.util.WallpaperUtils$2.onResponse(WallpaperUtils.java:263)
at com.afollestad.polar.util.WallpaperUtils$2.onResponse(WallpaperUtils.java:259)
at com.afollestad.bridge.RequestBuilder$9.response(RequestBuilder.java:492)
at com.afollestad.bridge.CallbackStack$1.run(CallbackStack.java:59)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)