Better error handling on passing invalid packs

src/package-manager.js

@@ -755,6 +755,10 @@ module.exports = class PackageManager {
   // Deactivate the package with the given name
   async deactivatePackage (name, suppressSerialization) {
     const pack = this.getLoadedPackage(name)
+    if (pack == null) {
+      return
+    }
+
     if (!suppressSerialization && this.isPackageActive(pack.name)) {
       this.serializePackage(pack)
     }

src/theme-manager.coffee

@@ -285,7 +285,8 @@ class ThemeManager
   deactivateThemes: ->
     @removeActiveThemeClasses()
     @unwatchUserStylesheet()
-    Promise.all(@packageManager.deactivatePackage(pack.name) for pack in @getActiveThemes())
+    results = @getActiveThemes().map((pack) => @packageManager.deactivatePackage(pack.name))
+    Promise.all(results.filter((r) => typeof r?.then is 'function'))
 
   isInitialLoadComplete: -> @initialLoadComplete