[gnome-shell] js/main: Throw error if no valid default stylesheet is found

[Marco Trevisan <marcotrevi src gnome org>]

commit 30cb2127a14bf291cd15e92111022d636d3c9a1f
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Wed Aug 15 03:33:13 2018 +0200

    js/main: Throw error if no valid default stylesheet is found
    
    Throw an error using an informative message in case a mode uses a stylesheet
    that can't be loaded, instead of crashing later because the theming can't be
    properly computed, and thus the minimum size of the actors.

 js/ui/main.js | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/js/ui/main.js b/js/ui/main.js
index e107de744..9e7ce8706 100644
--- a/js/ui/main.js
+++ b/js/ui/main.js
@@ -343,6 +343,9 @@ function loadTheme() {
     let theme = new St.Theme ({ application_stylesheet: _cssStylesheet,
                                 default_stylesheet: _defaultCssStylesheet });
 
+    if (theme.default_stylesheet == null)
+        throw new Error("No valid stylesheet found for '%s'".format(sessionMode.stylesheetName));
+
     if (previousTheme) {
         let customStylesheets = previousTheme.get_custom_stylesheets();