[bugfix] [cpp-ue4] API often tries to set reference to null when calling GetHttpRetryManager (#10277)

* [cpp-ue4] implemented null check and fatal log for GetHttpRetryManager]

* [cpp-ue4] reimplemented null check as checkf rather than Fatail UE_LOG

* [cpp-ue4] regenerated samples and docs

modules/openapi-generator/src/main/resources/cpp-ue4/api-source.mustache

@@ -56,6 +56,7 @@ void {{classname}}::SetHttpRetryManager(FHttpRetrySystem::FManager& InRetryManag
 
 FHttpRetrySystem::FManager& {{classname}}::GetHttpRetryManager()
 {
+	checkf(RetryManager, TEXT("{{classname}}: RetryManager is null.  You may have meant to set it with SetHttpRetryManager first, or you may not be using a custom RetryManager at all."))
 	return *RetryManager;
 }
 

samples/client/petstore/cpp-ue4/Private/OpenAPIPetApi.cpp

@@ -65,6 +65,7 @@ void OpenAPIPetApi::SetHttpRetryManager(FHttpRetrySystem::FManager& InRetryManag
 
 FHttpRetrySystem::FManager& OpenAPIPetApi::GetHttpRetryManager()
 {
+	checkf(RetryManager, TEXT("OpenAPIPetApi: RetryManager is null.  You may have meant to set it with SetHttpRetryManager first, or you may not be using a custom RetryManager at all."))
 	return *RetryManager;
 }
 

samples/client/petstore/cpp-ue4/Private/OpenAPIStoreApi.cpp

@@ -65,6 +65,7 @@ void OpenAPIStoreApi::SetHttpRetryManager(FHttpRetrySystem::FManager& InRetryMan
 
 FHttpRetrySystem::FManager& OpenAPIStoreApi::GetHttpRetryManager()
 {
+	checkf(RetryManager, TEXT("OpenAPIStoreApi: RetryManager is null.  You may have meant to set it with SetHttpRetryManager first, or you may not be using a custom RetryManager at all."))
 	return *RetryManager;
 }
 

samples/client/petstore/cpp-ue4/Private/OpenAPIUserApi.cpp

@@ -65,6 +65,7 @@ void OpenAPIUserApi::SetHttpRetryManager(FHttpRetrySystem::FManager& InRetryMana
 
 FHttpRetrySystem::FManager& OpenAPIUserApi::GetHttpRetryManager()
 {
+	checkf(RetryManager, TEXT("OpenAPIUserApi: RetryManager is null.  You may have meant to set it with SetHttpRetryManager first, or you may not be using a custom RetryManager at all."))
 	return *RetryManager;
 }