Call target.onload() only when defined

[Javarome]

Brief description of the changes

This PR checks that the onload function is defined before calling it. This removes the Uncaught TypeError: target.onload is not a function console error while img preview.

What browsers and operating systems have you tested these changes on?

Chrome 69.0.3497.100 on MacOS Sierra 10.13.6

Have you written unit tests? If not, explain why.

No, because the modified code is likely already tested.

[singhjusraj]

Hi @Javarome
When does the Uncaught TypeError: target.onload is not a function error actually comes?
If you could let me know the reproduction steps, I would like to see the error.

[Javarome]

Hi @SinghSukhdeep

I encountered it many times in my production code when using <img> for preview. I am unable to share it here, but another user has reported here with some sample code : #1882

A PR was rejected about it because it prevented calling onload when defined, whereas this current PR will avoid calling it only when undefined.

In any case such a change can only make the code more robust by avoiding failure when the callback is not defined (but still calling it when it is).

[singhjusraj]

Okay sounds good. Will merge this and release with few other pending PRs