First of all, congrats on being invited to speak! Would it be possible for you to give a broad review of the state of the field in your talk? This should perhaps be run by the conference organizers who invited you, to see if this is something they would be interested in. But your comment about:
The theme is exactly related on my doctoral work, and they really insisted on wanting my expertise because not many people are working specifically on this topic
makes me think that your insights on the state of the field might be precisely what the organizers are interested in. And in reviewing the state-of-the-art, presumably your masters work would be one example, but not the entirety of the talk.
I think this suggestion:
My advisor suggested that I could talk about the context/background for my PhD research.
would also fit nicely in such a talk.
Regarding your concern:
I'm hesitant to jump into the methodology part because I don't want this idea to get scooped since I am such in the early stages of my doctoral program
if you choose to do a "review" talk, then I think you can focus on what you see as the remaining problems in the field, and then see how much of your intended methodology seems to be "trivial" or "obvious" given those problems, since it's sharing publicly the "nontrivial" ideas that you might need to worry about. Certainly if there's anything patentable in your methodology, you should discuss with your advisor how to go about protecting the ideas before the talk.
Otherwise, I think presenting (and claiming!) the ideas at the conference might actually be helpful for your PhD work. For instance, you might have some attendees ask you if they can collaborate with you on your ideas, and/or you might get some useful feedback on them (e.g., "have you seen this relevant paper?", "did you consider this challenge?", etc.).
