resolves #4270 don't raise error if Asciidoctor::Extensions.unregiste…

…r is called before groups are initialized

CHANGELOG.adoc

@@ -19,6 +19,7 @@ This project utilizes semantic versioning.
 
 Bug Fixes::
 
+  * Don't raise error if `Asciidoctor::Extensions.unregister` is called before groups are initialized (#4270)
   * If path is included both partially and fully, store it with true value (included fully) in includes table of document catalog
   * Reset registry if activate is called on it again (#4256)
   * Format source location in exception message when extension code is malformed

lib/asciidoctor/extensions.rb

@@ -1540,7 +1540,7 @@ def unregister_all
     #
     # Returns nothing
     def unregister *names
-      names.each {|group| @groups.delete group.to_sym }
+      names.each_with_object(groups) {|group, catalog| catalog.delete group.to_sym }
       nil
     end
   end

test/extensions_test.rb

@@ -344,6 +344,19 @@ def create_santa_list_block_macro
       end
     end
 
+    test 'should not fail to unregister extension group if not registered' do
+      refute_nil Asciidoctor::Extensions.groups
+      assert_equal 0, Asciidoctor::Extensions.groups.size
+      Asciidoctor::Extensions.unregister :sample
+      assert_equal 0, Asciidoctor::Extensions.groups.size
+    end
+
+    test 'should not fail to unregister extension group if extension groups are not initialized' do
+      Asciidoctor::Extensions.remove_instance_variable :@groups
+      Asciidoctor::Extensions.unregister :sample
+      assert_equal 0, Asciidoctor::Extensions.groups.size
+    end
+
     test 'should raise NameError if extension class cannot be resolved from string' do
       begin
         Asciidoctor::Extensions.register do