App: Fix crash when the express number is too short

[liuweiky]

Some users may attempt to add an irregular number, which will lead to the crash as follows.

07-15 22:40:45.652 7322-7322/info.papdt.express.helper E/AndroidRuntime: FATAL EXCEPTION: main
    Process: info.papdt.express.helper, PID: 7322
    java.lang.StringIndexOutOfBoundsException: length=1; index=4
        at java.lang.String.substring(String.java:1971)
        at info.papdt.express.helper.ui.fragment.add.AddDialogFragment$createView$8.onClick(AddDialogFragment.kt:113)
        at android.view.View.performClick(View.java:6303)
        at android.view.View$PerformClick.run(View.java:24828)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6798)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)