You have attempted to open an incompatible Pages document created by Pages v5 — v7.3 in Pages '09. The dialog "the required index.xml is missing,” is the result, and only produced by Pages '09. This particular message has nothing to do about Pages crashing.
Ensure that you quit Pages '09 from its application menu (or cmd+Q) if it is still running. If you have a version of Pages in your Application folder with the following icon:

then you can press and hold the shift key while launching this version of Pages. Then open the same document from the first paragraph that caused the index.xml dialog in Pages '09. If it is not irreparably damaged, it should open correctly in the newer version of Pages. If you do not have a yellow Pages icon in your Applications folder, you will only be able to open these newer documents with Pages for iCloud.
There is a Pages for Mac Support Community available to you.