It depends on whether the angle of thesis topic selection is biased towards procedural issues or focuses on substantive issues. If it is the former, you can choose the civil procedure law; If it is the latter, civil law can be chosen.

The reason is that the third party without independent claim refers to the person who has legal interest in the outcome of the case, although he has no independent claim to the disputed litigation, so he has participated in the litigation that has already begun. The third party without independent claim is a participant in civil litigation, which is inextricably linked with the civil procedure law. If we want to demonstrate the procedural issues such as the way the third party participates in litigation and how to remedy the rights, we should choose the civil procedure law. If we want to demonstrate the rights and obligations of third parties and other substantive issues, the voter law.

Personally, I think it is better to write a thesis if I choose the direction of civil procedure law.