Fixes thrown exceptions in apps using postMessage

[jacwright]

My app uses postMessage for it's own purposes. This fixes an error that occurs when the appcache updates choke on messages from the application.

[NekR]

Hi @jacwright, seems like a right thing to do, but interesting why it passes this check:

https://github.com/NekR/offline-plugin/blob/master/tpls/runtime-template.js#L144

[jacwright]

You have a good point. So I looked into it a bit more and found that FullStory (which my app uses) sends messages to each iframe to try and "record" it. The message coming through is one from full-story which doesn't match what the appcache expects. Definitely an edge case, but I suppose still nice to make it more robust against error.

[NekR]

I see. But AppCache's iframe is the one sending messages and main window is accepting them. It checks and accepts messages from AppCache's iframe only. So the only situation where you fix may help is if AppCache's iframe will be sending incorrect data (which is unexpected of course). Are you sure that's what happens? Also, are you sure the proposed fix fixes your situation?

[jacwright]

1. all this fix does is a null-check, which fixes my error because I'm getting them thrown in my console
2. FullStory injects its script into the iframe and sends messages up to the main window where it is collating all changes to the DOM in and out of iframes. This is a feature of FullStory (not one you can turn on/off) that lets it record all changes to the page, even within iframes that aren't sandboxed or that also have fullstory loaded within them. It's quite nice.

To be able to continue to use FullStory and offline-plugin I need this null check added. It is good defensive programming and will strengthen the project.

[NekR]

@jacwright I'm not trying to not merge this code. Just trying to fully understand why it's needed. Now I do.

There is no hard in it indeed, but next time someone asks why it's there it's good to have this conversation around :-) Thanks for explanation!

[NekR]

Oh, I'll make npm release a bit later today.

[NekR]

Sorry, I forgot to make a release yesterday, released now!