$$
\begin{align}
\int_0^{\pi/2}\int_0^{\sin(x)}\frac{\arcsin(t)}{t}\,\mathrm{d}t\,\mathrm{d}x
&=\int_0^{\pi/2}\int_0^xt\cot(t)\,\mathrm{d}t\,\mathrm{d}x\\
&=\int_0^{\pi/2}x(\pi/2-x)\cot(x)\,\mathrm{d}x\\
&=\int_0^{\pi/2}x(\pi/2-x)\tan(x)\,\mathrm{d}x\\
&=-\int_0^{\pi/2}x(\pi/2-x)\,\mathrm{d}\log(\cos(x))\\
&=\int_0^{\pi/2}\log(\cos(x))(\pi/2-2x)\,\mathrm{d}x\\
&=\int_0^{\pi/2}\left(-\log(2)+\sum_{k=1}^\infty(-1)^{k-1}\frac{\cos(2kx)}{k}\right)(\pi/2-2x)\,\mathrm{d}x\\
&=\int_0^{\pi/2}\sum_{k=1}^\infty(-1)^{k-1}\frac{\cos(2kx)}{k}(\pi/2-2x)\,\mathrm{d}x\\