Check for null thread groups at initialization

Change-Id: Ia6fffe9f416ac4bd2588016b99b172cf023f3c5d
diff --git a/src/runtime.h b/src/runtime.h
index c583ab1..544dcf4 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -141,16 +141,10 @@
   static void Abort();
 
   // Returns the "main" ThreadGroup, used when attaching user threads.
-  jobject GetMainThreadGroup() const {
-    CHECK(main_thread_group_ != NULL || IsCompiler());
-    return main_thread_group_;
-  }
+  jobject GetMainThreadGroup() const;
 
   // Returns the "system" ThreadGroup, used when attaching our internal threads.
-  jobject GetSystemThreadGroup() const {
-    CHECK(system_thread_group_ != NULL || IsCompiler());
-    return system_thread_group_;
-  }
+  jobject GetSystemThreadGroup() const;
 
   // Attaches the calling native thread to the runtime.
   void AttachCurrentThread(const char* thread_name, bool as_daemon, jobject thread_group);