fix: add null checks for context in GET function to prevent errors

[gusongen]

Fix bug when context from state.vscdb is null.

CatchallRenderRequest (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/next-server.js:272:17)
    at async DevServer.handleRequestImpl (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/base-server.js:817:17)
    at async /home/bj-01/cursor-chat-browser/node_modules/next/dist/server/dev/next-dev-server.js:339:20
    at async Span.traceAsyncFn (/home/bj-01/cursor-chat-browser/node_modules/next/dist/trace/trace.js:154:20)
    at async DevServer.handleRequest (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/dev/next-dev-server.js:336:24)
    at async invokeRender (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/lib/router-server.js:173:21)
    at async handleRequest (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/lib/router-server.js:350:24)
    at async requestHandlerImpl (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/lib/router-server.js:374:13)
    at async Server.requestListener (/home/bj-01/cursor-chat-browser/node_modules/next/dist/server/lib/start-server.js:141:13)