GH-1853 fix FTB and parsing of old version files

api/logic/minecraft/ftb/FTBProfileStrategy.cpp

@@ -59,6 +59,7 @@ void FTBProfileStrategy::loadDefaultBuiltinPatches()
 
 			// adapt the loaded file - the FTB patch file format is different than ours.
 			file->minecraftVersion.clear();
+			file->mainJar = nullptr;
 			for(auto addLib: file->libraries)
 			{
 				addLib->setHint("local");

api/logic/minecraft/onesix/OneSixVersionFormat.cpp

@@ -160,6 +160,7 @@ VersionFilePtr OneSixVersionFormat::versionFileFromJson(const QJsonDocument &doc
 		{
 			lib->setAbsoluteUrl(URLConstants::getLegacyJarUrl(out->minecraftVersion));
 		}
+		out->mainJar = lib;
 	}
 
 	/* removed features that shouldn't be used */