[in_app_purchase] Fix crash iOS sandboxTesting default value

[Vardiak]

Description

Currently, in_app_purchase crashes when initiating a payment if sandboxTesting of PurchaseParam is null. But it is actually its default value (contrary to the definition which states that default value is false). In this pull request, the default value is fixed & I added an assert to check if the user didn't pass null explicitly which would cause a crash.

Related Issues

flutter/flutter#65767

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[Vardiak]

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[cyanglaz]

@Vardiak Thanks for the fix! Could you add a test for this?

In order to make it non-public API change fix, I would recommend to fix it on the OBJC side. We can always allow this value to be null and in the OBJC side, we just take the null value as false.

[Vardiak]

@Vardiak Thanks for the fix! Could you add a test for this?

In order to make it non-public API change fix, I would recommend to fix it on the OBJC side. We can always allow this value to be null and in the OBJC side, we just take the null value as false.

I'm not a OBJC developer, I may be able to help fix it there but I don't know what kind of test you are expecting me to write, since the fix is straightforward.

[cyanglaz]

@Vardiak The test can be "if the sandboxTesting is nil, there is no crash"
I think there some OBJC unit tests in the plugin that you can follow.

I can also take over this fix if you don't feel like learning a bit of OBJC :) However, I'm currently working on some other high priority stuff so it might take some time before I can cycle back to this.

[Vardiak]

#3110 handles the issue at platform level, which is a better implementation