Handle missing project workspace diff base ref

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/pages/ProjectWorkspaceDetail.test.tsx

@@ -235,4 +235,22 @@ describe("ProjectWorkspaceDetail changes tab", () => {
       "/projects/paperclip-app/workspaces/workspace-1?tab=changes&diffView=head&baseRef=origin%2Fmain",
     );
   });
+
+  it("opens the Changes tab in working-tree mode when no upstream ref is known", async () => {
+    mockProjectsApi.get.mockResolvedValue(project({
+      workspaces: [projectWorkspace({ defaultRef: null, repoRef: null })],
+      primaryWorkspace: projectWorkspace({ defaultRef: null, repoRef: null }),
+      codebase: null,
+    }));
+
+    await render();
+
+    await act(async () => {
+      (container.querySelector('[data-tab-value="changes"]') as HTMLButtonElement).click();
+    });
+
+    expect(mockNavigate).toHaveBeenCalledWith(
+      "/projects/paperclip-app/workspaces/workspace-1?tab=changes&diffView=working-tree",
+    );
+  });
 });

ui/src/pages/ProjectWorkspaceDetail.tsx

@@ -428,11 +428,11 @@ export function ProjectWorkspaceDetail() {
   const handleTabChange = (tab: ProjectWorkspaceTab) => {
     const workspacePath = projectWorkspaceUrl(project, routeWorkspaceId);
     if (tab === "changes") {
+      const baseRef = upstreamBaseRef(workspace);
       const params = new URLSearchParams({
         tab: "changes",
-        diffView: "head",
+        diffView: baseRef ? "head" : "working-tree",
       });
-      const baseRef = upstreamBaseRef(workspace);
       if (baseRef) params.set("baseRef", baseRef);
       navigate(`${workspacePath}?${params.toString()}`);
       return;