Fixes issue #9 (failure when no 'Activities' present).

src/extension.js

@@ -117,7 +117,7 @@ WeatherMenuButton.prototype = {
         // Panel icon
         this._weatherIcon = new St.Icon({
             icon_type: this._icon_type,
-            icon_size: Main.panel.button.get_child().height,
+            icon_size: Main.panel.actor.get_theme_node().get_height() - 2,
             icon_name: 'view-refresh-symbolic',
             style_class: 'weather-icon' + (Main.panel.actor.get_direction() == St.TextDirection.RTL ? '-rtl' : '')
         });