fix: route home for missing problems

client/pages/Problem.tsx

@@ -1,6 +1,6 @@
 import { route } from 'preact-router'
 import { useContext, useState } from 'preact/hooks'
-import { isAdministrator, Solution as SolutionModel } from '../../shared/model'
+import { isAdministrator, Problem as ProblemModel, Solution as SolutionModel } from '../../shared/model'
 import { prependBaseUrl, server } from '../api'
 import { Header } from '../components/Header'
 import { MarkdownEditor } from '../components/MarkdownEditor'
@@ -21,15 +21,13 @@ export const ProblemPage = ({ id }: RouteProps) => {
 
     const [source, setSource] = useState('')
 
-    const [{ content }] = useResource<{ content?: string }>(
-        `/api/problem/${id}`,
-        {
-            content: '',
-        },
-        () => {
+    const [problem] = useResource<ProblemModel | null>(`/api/problem/${id}`, null, problem => {
+        if (problem === null) {
             route(prependBaseUrl('/'))
         }
-    )
+    })
+
+    const content = problem?.content ?? ''
 
     const [solutions, refreshSolutions, setSolutionHeuristic] = useListResource<SolutionModel>(
         `/api/solutions?problem=${id}`