Solution (source code)

= Solution

Since $\mathbb EN_t=\operatorname{Var}(N_t)=\lambda t$, use the <Poisson moment-generating function> to get, for $t>0$,
$$
\boxed{M_{U_t}(\theta)=\exp\left\{\lambda t\left(e^{\theta/\sqrt{\lambda t}}-1-\frac\theta{\sqrt{\lambda t}}\right)\right\}}.
$$
This is finite for every real $\theta$.