  We discuss a quantum transition from a superfluid to a Mott glass phases in
disordered Bose-systems by the example of an isotropic spin-$\frac12$
antiferromagnet with spatial dimension $d\ge2$ and with disorder in tunable
exchange couplings. Our analytical consideration is based on general properties
of a system in critical regime, on the assumption that the magnetically order
part of the system shows fractal properties near the transition, and on a
hydrodynamic description of long-wavelength magnons in the magnetically ordered
("superfluide") phase. Our results are fully consistent with a scaling theory
based on an ansatz for the free energy proposed by M.P. Fisher et al. (Phys.
Rev. B 40, 546 (1989)). We obtain $z=d-\beta/\nu$ for the dynamical critical
exponent and $\phi = z\nu$, where $\phi$, $\beta$, and $\nu$ are critical
exponents of the critical temperature, the order parameter, and the correlation
length, respectively. The density of states of localized excitations (fractons)
is found to show a superuniversal (i.e., independent of $d$) behavior.
