Fix failing test

[jjwesse]

UI test fails

 77 passing (46ms)
  1 failing
  1) UI component
       should generate appropriate output:
     TypeError: Cannot read properties of undefined (reading 'header')
      at Header.view (src/ui/Header.js:26:45)
      at renderComponent (node_modules/mithril-query/index.js:160:31)
      at renderNode (node_modules/mithril-query/index.js:176:9)
      at /home/runner/work/cmap-js/cmap-js/node_modules/mithril-query/index.js:169:16
      at Array.map (<anonymous>)
      at renderNode (node_modules/mithril-query/index.js:168:19)
      at renderNode (node_modules/mithril-query/index.js:182:31)
      at renderNode (node_modules/mithril-query/index.js:174:14)
      at Object.redraw (node_modules/mithril-query/index.js:204:22)
      at scan (node_modules/mithril-query/index.js:207:7)
      at init (node_modules/mithril-query/index.js:424:11)
      at Context.<anonymous> (test/ui/UI_test.js:7:15)
      at processImmediate (node:internal/timers:466:21)