Cannot load api via jquery getScript()

### Expected Behaviour

Use jquery getScript() to load the API at run time when needed, the script is loaded and the `adobe_dc_view_sdk.ready` event is fired.

### Actual Behaviour

An unhandled javascript exception occurs: `TypeError: Cannot read properties of undefined (reading 'src') at loadSDKScript`

#### Sample Code that illustrates the problem

https://jsfiddle.net/redtopia/o3m689sg/6/ (view the console to see the exception)

The following fiddle fixes the issue by loading the script with vanilla javascript

https://jsfiddle.net/redtopia/Lyqdj2zm/6/