Phase plane (I can't use quiver)

I have this system myth a simple PID controller

u = -Kp*(x(1) - theta_d) - Ki*(x(3)) - Kd*(x(2));

dxdt(1) = x(2);
dxdt(2) = (-l*Fg*sin(x(1)))/I - (l*x(2))/I + (g*l*u)/I;
dxdt(3) = x(1)-theta_d;


dxdt = dxdt';

but I can't to apply the quiver command, the figure don´t show anything.

Can someone help me please. Thanks, regards!