Fixed migrations, making all them reversible, so it's possible to uninstall plugin now by rake redmine:plugins:migrate NAME=redmine_custom_workflows VERSION=0

app/models/custom_workflow.rb

@@ -90,6 +90,7 @@ class CustomWorkflow < ActiveRecord::Base
   end
 
   def validate_syntax
+    return unless respond_to? :observable
     case observable
       when 'shared'
         CustomWorkflow.run_shared_code(self)

db/migrate/20120601054047_alter_custom_workflows.rb

@@ -8,6 +8,5 @@ class AlterCustomWorkflows < ActiveRecord::Migration
   end
 
   def self.down
-    raise ActiveRecord::IrreversibleMigration
   end
 end

db/migrate/20120628060102_change_custom_workflows_description_type.rb

@@ -4,6 +4,5 @@ class ChangeCustomWorkflowsDescriptionType < ActiveRecord::Migration
   end
 
   def self.down
-    change_column :custom_workflows, :description, :string, :null => false, :default => ''
   end
 end

db/migrate/20150522134437_set_position_field_nullable.rb

@@ -1,5 +1,7 @@
 class SetPositionFieldNullable < ActiveRecord::Migration
-  def change
+  def up
     change_column :custom_workflows, :position, :integer, :null => true
   end
+  def down
+  end
 end