Make sure we always have a stack (#134)

* Make sure we always have a stack

* cs

* style update

Author: Nyholm

Collector/Collector.php

@@ -62,7 +62,7 @@ public function getStacks()
     }
 
     /**
-     * @return Stack
+     * @return Stack|bool false if no current stack.
      */
     public function getCurrentStack()
     {

Collector/ProfilePlugin.php

@@ -58,7 +58,9 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
     {
         $profile = new Profile($this->pluginName, $this->formatter->formatRequest($request));
 
-        $this->collector->getCurrentStack()->addProfile($profile);
+        if ($stack = $this->collector->getCurrentStack()) {
+            $stack->addProfile($profile);
+        }
 
         return $this->plugin->handleRequest($request, $next, $first)->then(function (ResponseInterface $response) use ($profile) {
             $profile->setResponse($this->formatter->formatResponse($response));