android问题求教

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mymail/com.example.mymail.AddcontantsActivity}: java.lang.IllegalArgumentException: the bind value at index 1 is null

在点击按钮用对话框添加邮件联系人时候报错，第二次点击后就连界面都无法启动了。

 public void addcontants(){
 AlertDialog.Builder builder=new Builder(AddcontantsActivity.this);
 Context mContext = AddcontantsActivity.this;
 LayoutInflater inflater = getLayoutInflater();

 mContext.getSystemService(LAYOUT_INFLATER_SERVICE);
 View layout = inflater.inflate(R.layout.mycustom_dialog,null);
 final EditText name=(EditText) layout.findViewById(R.id.addcontantsname);
 final EditText addr=(EditText) layout.findViewById(R.id.addcontantsemail);
        System.out.println(addr_str);

        builder.setView(layout);
        builder.setPositiveButton("确定", new OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                name_str=name.getText().toString().trim();
                addr_str=addr.getText().toString().trim();
                System.out.println(name_str+"////"+addr_str);

                if(TextUtils.isEmpty(name_str)){
                    Toast.makeText(AddcontantsActivity.this, "用户名不能为空", Toast.LENGTH_SHORT).show();
                }else{
                    if(!Emailcheck.isEmail(addr_str)){

                        Toast.makeText(AddcontantsActivity.this, "邮箱格式不正确", Toast.LENGTH_SHORT).show();
                    }else{
                        Uri uri=Uri.parse("content://com.example.mymail.provider.EmailConstantProv");
                        ContentValues values=new ContentValues();
                        values.put("mailfrom", MyApplication.info.getUserName());
                        values.put("name", name_str);
                        values.put("address", addr_str);
                        getContentResolver().insert(uri, values);

                        Toast.makeText(AddcontantsActivity.this, "添加数据成功", Toast.LENGTH_SHORT).show();
                    }
                 }
            }
        });
        builder.setNegativeButton("取消", null);
        builder.show();
}

数据库信息

db.execSQL("create table email(id INTEGER PRIMARY KEY AUTOINCREMENT,mailfrom varchar(20),name varchar(20),address varchar(20))");