Initialization of VertexBuffer is done by call from BaseContext form its constructor :

public abstract class BaseContext {
    private final VertexBuffer vertexBuffer;
    BaseContext() {
       this.vertexBuffer = createVertexBuffer();
    }

while createVertexBuffer is a vistual function wich is overriden and directs to un-initialized child class D3DContext.
As a result we have an un-initialized D3DContext in the D3DVertexBuffer(D3DContext context...) constructor.
This needs to be re-done in a proper way.
We need to remove virtual calls from constructors.