$X_{1}, X_{2}, \ldots, X_{n}$ form a random sample from a distribution whose probability density function is

$$f(x ; \theta)=\left\{\begin{array}{cc} \frac{2 x}{\theta^{2}} & 0 \leqslant x \leqslant \theta \\ 0 & \text { otherwise } \end{array}\right.$$

where the value of the positive parameter $\theta$ is unknown. Determine the maximum likelihood estimator of the median of this distribution.