[bugfix] [cpp-ue4] API often tries to set reference to null when calling GetHttpRetryManager

Created by: leith-bartrich

fixes #10223 (closed)

As explained in the original bug report, there is undefined behavior in the GetHttpRetryManager generated code. This fix does a quick null check before the assignment, and emits an unreal Fatal log error if the code is about to do something undefined.

As explained in the bug report, there are other issues surrounding the data field in question. This bug fix does not seek to address those issues.

Rather, it simply seeks to allow a developer to get useful feedback and to crash out at the right time, rather than the wrong time.

Before this fix, an access violation was likely to occur further down the execution in some arbitrary place when a developer tried to use the undefined reference returned by the function. The stack trace at that time doesn't really point to the culprit.

After this fix, the developer gets a stack trace and failure at the actual point of undefined behavior. The stack trace goes through their code at the point where they are making a call that generates a null reference.

There are all kinds of potential changes to be made here. Such as a "HttpRetryManagerIsSet()" function. Or returning a pointer rather than a reference, which can be null.

Further, I'd think based on the implementation that the methods might be better titled, "SetCustomHttpRetryManager" "GetCustomHttpRetryManager" and "CustomHttpRetryManagerIsSet()"

However, all of those things are beyond the scope of this simple change. And if the underlying issues end up fixed I'd expect the need for this Fatal log entry to disappear.

In the interim, I'd expect some frustration from downstream developers who don't understand why the RetryManager is null when they call the Getter. But that's better than having them waste hours figuring out that the reference they're being handed is undefined when it's generated, and then discovering that it's because the internal RetryManager was null.

@Kahncode

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