getting nullpointerexception when trying to invoke method from another class even object was not null

Question

I created three classes A,B and C in the class A declared class B and class C as references in the constructor A passed the B and C references by using the B's object reference invoked the function which is in class B it worked fine. But did the same thing with class B and invoked class C function getting runtime exception. Please Refer to the below code. Can anyone give clear explanation why the problem is occurring how to resolve it.

class A
{
    B b;
    C c;
    A(B b,C c)
    {
        this.b=b;
        this.c=c;
        System.out.println("From Constructor Class A");
        System.out.println(c);
    }
    void display()
    {
        b.displayB();
    }   
}
class B
{
    C c;
    B(C c)
    {
        this.c=c;
        System.out.println("From Constructor C"+c); 
    }
    B()
    {   
    }
    void displayB()
    {
        System.out.println(c);
        System.out.println("From Class B");
        c.displayC();
    }
}
class C
{
    C()
    {
    }
    void displayC()
    {
        System.out.println("From Class C");
    }
}
class Doubt
{
    public static void main(String[] args)
    {
        B b1=new B();
        C c1=new C();
        System.out.println(c1);
        A a=new A(b1,c1);
        B b2=new B(c1);
        a.display();
        
                
    }
}