src: clear envinst_ after env

As long as env() doesn't return nullptr it is assumed that access to
envinst_ is possible. So change the order of cleanup.

src/nsolid/nsolid_api.cc

@@ -968,9 +968,6 @@ void EnvList::RemoveEnv(Environment* env) {
   envinst_sp->env()->isolate()->RemoveGCEpilogueCallback(
       EnvInst::v8_gc_epilogue_cb_, envinst_sp.get());
 
-  // Remove the instance from env
-  env->envinst_.reset();
-
   // End any pending spans and notify the user.
   if (env->is_main_thread()) {
     tracer_.endPendingSpans();
@@ -992,6 +989,9 @@ void EnvList::RemoveEnv(Environment* env) {
   // and only accessed from another thread if the env still exists and has been
   // Scope locked.
   envinst_sp->env_.store(nullptr, std::memory_order_relaxed);
+
+  // Remove the instance from env
+  env->envinst_.reset();
 }