Cannot use `bpmnjs.on` in module constructor when eagerly loaded

Describe the Bug

Given I have a module of the following structure:

const module = {
  __init__: [ 'custom' ],
  custom: [ 'type', Custom ]
}

function Custom(bpmnjs) {
  bpmnjs.on('diagram.init' /** can be any event */, () => console.log('hello.world'))
}

when I load it to bpmn-js, I get an error similar to the one below:

TypeError: this.get is not a function
    at Modeler.BaseViewer.on (BaseViewer.js?b392:559:1)
    at new ExecutionPlatform (execution-platform.js?5062:41:1)

This is due to injector (and modules) being instantiated before Diagram#get API is defined, cf. https://github.com/bpmn-io/diagram-js/blob/develop/lib/Diagram.js#L139 The BaseViewer tries to get the EventBus but get is not available yet.

Steps to Reproduce

See above.

Expected Behavior

No error. We could move the API declaration to the root of the module and use Diagram.prototype.get, but I haven't tested that.

Environment

• Browser: [e.g. IE 11, Chrome 69]
• OS: [e.g. Windows 7]
• Library version: [e.g. 2.0.0]