I still do not, exactly, know my field of interest. Is it relevant while applying for PhD? I am in my fourth year of BS-MS course in Physics in India. I will be applying for PhD next year but I do not have a specific topic in my mind. 

Until now I have done 2 projects on Cosmology (related to Dark Matter in specific). Right now I have started a project on Semi-Classical gravity, which has been mostly reading work until now. I have liked both topics. I am also being introduced to QFT, Particle Physics this year which also look interesting.

So I do not know what should do my PhD in. Is it necessary for me to pin point my topic of my interest within next 1 year? or Is it fine to have  "Theoretical Physics" as my topic of my interest?

As of now I planning to do my MS thesis on Semi-Classical Gravity so that I'll be able to explore a new topic as well as do something that I like at the same time. Is it necessary for me to decide my Master's thesis based on my potential PhD topic?
I know this field.  Theoretical is enough of a preference for now.  Realistically there are many different areas in physics that rely on conceptual and mathematical ability.  At least you know you don't want to be an experimenter.  If you did, I would think there was a little more importance in having at least an initial hypothesis on what area to go into.  But I think theory is a more flexible, especially for someone new.

P.s.  Don't sleep on theoretical solid state.  E.g. density functional theory.