Error - com.google.gson.JsonNull cannot be cast to com.google.gson.JsonObject

If loadSavedData is activated before any(first) saving, encounter following error message "com.google.gson.JsonNull cannot be cast to com.google.gson.JsonObject"

I've tried to fix but I couldn't...

Below is a problematic part

private void loadSavedData(){
    timetable.removeAll();
    SharedPreferences mPref = 
         PreferenceManager.getDefaultSharedPreferences(this);
    String savedData = mPref.getString("timetable_demo1","");
    if(savedData == null && savedData.equals("")) return;
    timetable.load(savedData);
    Toast.makeText(this,"loaded!",Toast.LENGTH_SHORT).show();
}

How can I skip loading-saved-data if savedData is null (initially operated. before the first saving something).. even though loadSavedData is activated in the above statement? (by adjust/refine the if statement.

No error under activating "loadSavedData" is required even though no data is saved before.