Segmentation fault with std::uniform_int_distribution

I have a program containing multiple classes, some of which require random doubles and ints. In one of the classes I defined a struct to seed the random engine and be able to generate random reals and ints through an object of that struct whenever I need one. The .hpp file with class and struct declations looks like this:

struct RNG {
public:
    static std::mt19937 gen;
    static std::uniform_int_distribution<uint32_t> dist_ui;
    static std::uniform_real_distribution<double> dist_d;
    uint32_t r_ui = dist_ui(gen);
    double r_d = dist_d(gen);
private:
    static unsigned seed;
};

class A {
private:
    uint32_t a_;
public:
    A();
};

The .cpp file looks like this:

#include "A.hpp"

unsigned RNG::seed = 42;
std::mt19937 RNG::gen(RNG::seed);
std::uniform_int_distribution<uint32_t> RNG::dist_ui(1, std::numeric_limits<uint32_t>::max());
std::uniform_real_distribution<double> RNG::dist_d(0.0,1.0);

A::A() {
    RNG* r;
    a_ = r->dist_ui(r->gen);
}

Now, if I call uniform_real_distribution with

r->dist_d(r->gen)

everthing works fine. However, if I call uniform_int_distribution with

r->dist_ui(r->gen)

as in the above snippet, I get a segmentation fault. The error also occurs if I define my struct only with int dist instead of both real and int, or if I change the boundaries of the int dist to [0,100) or whatever. Does anyone have a clue what happens here? I appreciate any help!