Error deactivating package 'minimap-pigments' TypeError: Cannot read property 'dispose' of undefined

Noticed this while developing another plugin, triggered when reloading the window:

```
/Applications/Atom.app/Contents/Resources/app.asar/src/package.js:655 Error deactivating package 'minimap-pigments' TypeError: Cannot read property 'dispose' of undefined
at Object.module.exports.deactivate (/Users/viddo/.atom/packages/minimap-pigments/lib/minimap-pigments.coffee:24:25)
at Package.module.exports.Package.deactivate (/Applications/Atom.app/Contents/Resources/app.asar/src/package.js:644:21)
at PackageManager.module.exports.PackageManager.deactivatePackage (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:622:12)
at /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:608:19
at Config.module.exports.Config.transact (/Applications/Atom.app/Contents/Resources/app.asar/src/config.js:312:16)
at PackageManager.module.exports.PackageManager.deactivatePackages (/Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:602:19)
at AtomEnvironment.module.exports.AtomEnvironment.unloadEditorWindow (/Applications/Atom.app/Contents/Resources/app.asar/src/atom-environment.js:799:21)
at WindowEventHandler.module.exports.WindowEventHandler.handleWindowBeforeunload (/Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:196:30)
at /Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:3:61module.exports.Package.deactivate @ /Applications/Atom.app/Contents/Resources/app.asar/src/package.js:655module.exports.PackageManager.deactivatePackage @ /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:622(anonymous function) @ /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:608module.exports.Config.transact @ /Applications/Atom.app/Contents/Resources/app.asar/src/config.js:312module.exports.PackageManager.deactivatePackages @ /Applications/Atom.app/Contents/Resources/app.asar/src/package-manager.js:602module.exports.AtomEnvironment.unloadEditorWindow @ /Applications/Atom.app/Contents/Resources/app.asar/src/atom-environment.js:799module.exports.WindowEventHandler.handleWindowBeforeunload @ /Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:196(anonymous function) @ /Applications/Atom.app/Contents/Resources/app.asar/src/window-event-handler.js:3
```

https://github.com/abe33/minimap-pigments/blob/master/lib/minimap-pigments.coffee#L24 `@editorsSubscription` does not always exist, it seems

The error occurs in lib/minimap-pigments.coffee at line 24, where @editorsSubscription.dispose is called but @editorsSubscription may be undefined. Check the deactivate method to see where @editorsSubscription is initialized, and add a guard condition before calling dispose. Run the plugin in Atom to verify the fix by reloading the window and ensuring no error appears in the developer console.

Written by the indexing model from the issue text.