尝试从Firebase数据库获取时无法获取CarType

Question

当我清除我的应用程序缓存时，当我尝试运行应用程序时，我收到以下错误它工作正常但是当我关闭它并再次运行它崩溃

Attempt to invoke virtual method 'java.lang.String com.ggsbusiness.amirmemon.comsatsbusservice.Model.ComsatsDriver.getCarType()' on a null object reference

当试图从Firebasedatabse获取当前的cartype时。

drivers = FirebaseDatabase.getInstance().getReference(Common.driver_tbl).child(Common.currentBusDriver.getCarType());

来自Common.class

public class Common {

    public static final String driver_tbl = "Drivers";
    public static final String user_driver_tbl = "DriversInformation";
    public static final String user_rider_tbl = "RidersInformation";
    public static final String pickup_request_tbl = "PickupRequest";


    public static final String baseURL = "https://maps.googleapis.com";
    public static ComsatsDriver currentBusDriver;

    public static IGoogleAPI getGoogleAPI()
    {
        return RetrofitClient.getClient(baseURL).create(IGoogleAPI.class);
    }
}

这是ComsatsDriver.class：

public class ComsatsDriver {
    private String name,phone,carType;

    public ComsatsDriver(){
    }

    public ComsatsDriver(String name, String phone, String carType) {
        this.name = name;
        this.phone = phone;
        this.carType = carType;
    }


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String getCarType() {
        return carType;
    }

    public void setCarType(String carType) {
        this.carType = carType;
    }
   }