fix: handle null values in Figma prototype file responses

[BillionClaw]

Problem

When accessing Figma prototype files (URLs with /proto/ format), the MCP server crashes with:

cannot read properties of null (reading 'components')

This occurs because:

1. The Figma API returns
   null
   for node responses when accessing prototype files or inaccessible branches
2. The
   parseAPIResponse()
   function in
   design-extractor.ts
   does not handle null values before accessing properties
3. Both
   GetFileNodesResponse
   and
   GetFileResponse
   paths lack null-safety checks

Fixes #201

Root Cause

In
src/extractors/design-extractor.ts
:

• Line 58:
  nodeResponse.components
  accessed without checking if
  nodeResponse
  is null
• Line 68:
  n.document
  accessed without optional chaining
• Line 74-75:
  data.components
  and
  data.componentSets
  accessed without null checks
• Line 80:
  data.document.children
  accessed without optional chaining

Changes

Added comprehensive null-safety checks:

1. Skip null nodeResponses (line 58): Added early return if
   nodeResponse
   is null
2. Optional chaining for document access (line 70): Changed to
   n?.document
   with proper filter
3. Null checks for components/componentSets (lines 74-79): Added
   if
   guards before Object.assign
4. Safe document children access (line 82): Changed to
   data.document?.children?.filter()
   with nullish coalescing to empty array

Verification

• ✅
  pnpm type-check
  : No TypeScript errors
• ✅
  pnpm test
  : All 11 tests pass (1 skipped)
• ✅
  git diff --check
  : No whitespace issues
• ✅ Changes limited to 1 file:
  src/extractors/design-extractor.ts
  (+11/-4 lines)

Risk/Edge Cases

• Prototype files: Now handled gracefully by skipping null responses
• Inaccessible branches: Same fix applies - null responses are filtered out
• Regular design files: No behavioral change - existing functionality preserved
• Empty responses: Returns empty
  nodesToParse
  array instead of crashing

Issue Link

Closes #201