Thinking about only calling static variables in java static methods

Recently I wrote a class like this:

public class DBManager {
    private static MySqliteHelper helper;
    public static MySqliteHelper getHelper(Context context){
        if (helper == null){
            helper = new MySqliteHelper(context);
        }
        return helper;
    }

    public static void execSql(SQLiteDatabase db,String sql){
        if (db != null){
            if (sql != null && !"".equals(sql)){
                db.execSQL(sql);
            }
        }
    }
}

private static MySqliteHelper helper;

At the beginning, I did not add static modification to the above line of code, and then the tool reported an error.

Let's analyze the reasons carefully: static variables and static methods are shared by the entire class, and can be called regardless of whether the class has instantiated an object or not.

A reference to a non-static variable must first initialize the object.

"".equals(sql)

In addition, the above line of code is written in this way to avoid null pointer exception.