How can I fix this dent on my nose bridge? How safe are injections after rhinoplasty? What exactly is the reason for it? (Photo)

In these limited photographs, your nasal bones appear much wider than your upper lateral cartilages, so the indentations are in disjunction between your upper lateral cartilage and your nasal bones themselves. A closed rhinoplasty approach can accomplish narrowing your bridge line, smoothing out any irregularities, and making your upper lateral cartilages and nasal bones in a more aligned fashion for better symmetry.

Thank you for your question. The irregularity on the bridge of your nose (from the photographs) appears to be secondary to an incomplete nasal fractures which give the nasal bridge a wide appearance. It is difficult to tell from the photograph but it appears you may also have some deviation of your nasal septum. This gives (from the photograph only) a C-shaped appearance.
I would recommend a surgical revision rhinoplasty possibly using an open technique.
It is possible to correct your nasal irregularity with a filler, but often times the amount of filler required may give your nose a much wider appearance.