Example site does not render with Hugohugo v0.71.1

How to repoduce:

1. Upgrade to Hugo v0.71.1
2. hugo server --source=exampleSite --themesDir="../../"

Output:

Building sites … WARNING: calling IsSet with unsupported type "string" (string) will always return false.

ERROR 2020/05/28 13:00:20 render of "page" failed: execute of template failed: template: _default/single.html:21:27: executing "header" at <partial "header.html" .>: error calling partial: execute of template failed: template: partials/header.html:15:5: executing "partials/header.html" at <partial "navbar.html" .>: error calling partial: execute of template failed: template: partials/navbar.html:78:13: executing "partials/navbar.html" at <partial "nav_toggle.html" .>: error calling partial: "/Users/chrisguindon/localhugo/hugo-solstice-theme/layouts/partials/nav_toggle.html:14:8": execute of template failed: template: partials/nav_toggle.html:14:8: executing "partials/nav_toggle.html" at <(ne .Site.Menus.main nil) and (ne (first .Site.Menus.main) nil)>: can't give argument to non-function ne .Site.Menus.main nil
ERROR 2020/05/28 13:00:20 render of "page" failed: execute of template failed: template: _default/single.html:21:27: executing "header" at <partial "header.html" .>: error calling partial: execute of template failed: template: partials/header.html:15:5: executing "partials/header.html" at <partial "navbar.html" .>: error calling partial: execute of template failed: template: partials/navbar.html:78:13: executing "partials/navbar.html" at <partial "nav_toggle.html" .>: error calling partial: "/Users/chrisguindon/localhugo/hugo-solstice-theme/layouts/partials/nav_toggle.html:14:8": execute of template failed: template: partials/nav_toggle.html:14:8: executing "partials/nav_toggle.html" at <(ne .Site.Menus.main nil) and (ne (first .Site.Menus.main) nil)>: can't give argument to non-function ne .Site.Menus.main nil
ERROR 2020/05/28 13:00:20 render of "page" failed: execute of template failed: template: _default/single.html:21:27: executing "header" at <partial "header.html" .>: error calling partial: execute of template failed: template: partials/header.html:15:5: executing "partials/header.html" at <partial "navbar.html" .>: error calling partial: execute of template failed: template: partials/navbar.html:78:13: executing "partials/navbar.html" at <partial "nav_toggle.html" .>: error calling partial: "/Users/chrisguindon/localhugo/hugo-solstice-theme/layouts/partials/nav_toggle.html:14:8": execute of template failed: template: partials/nav_toggle.html:14:8: executing "partials/nav_toggle.html" at <(ne .Site.Menus.main nil) and (ne (first .Site.Menus.main) nil)>: can't give argument to non-function ne .Site.Menus.main nil
ERROR 2020/05/28 13:00:20 render of "page" failed: execute of template failed: template: _default/single.html:21:27: executing "header" at <partial "header.html" .>: error calling partial: execute of template failed: template: partials/header.html:15:5: executing "partials/header.html" at <partial "navbar.html" .>: error calling partial: execute of template failed: template: partials/navbar.html:78:13: executing "partials/navbar.html" at <partial "nav_toggle.html" .>: error calling partial: "/Users/chrisguindon/localhugo/hugo-solstice-theme/layouts/partials/nav_toggle.html:14:8": execute of template failed: template: partials/nav_toggle.html:14:8: executing "partials/nav_toggle.html" at <(ne .Site.Menus.main nil) and (ne (first .Site.Menus.main) nil)>: can't give argument to non-function ne .Site.Menus.main nil
Built in 243 ms
Error: Error building site: failed to render pages: render of "page" failed: execute of template failed: template: _default/single.html:21:27: executing "header" at <partial "header.html" .>: error calling partial: execute of template failed: template: partials/header.html:15:5: executing "partials/header.html" at <partial "navbar.html" .>: error calling partial: execute of template failed: template: partials/navbar.html:78:13: executing "partials/navbar.html" at <partial "nav_toggle.html" .>: error calling partial: "/Users/chrisguindon/localhugo/hugo-solstice-theme/layouts/partials/nav_toggle.html:14:8": execute of template failed: template: partials/nav_toggle.html:14:8: executing "partials/nav_toggle.html" at <(ne .Site.Menus.main nil) and (ne (first .Site.Menus.main) nil)>: can't give argument to non-function ne .Site.Menus.main nil