Make sure $stack is null or Stack. Never boolean.

Collector/Collector.php

@@ -62,11 +62,15 @@ public function getStacks()
     }
 
     /**
-     * @return Stack|bool false if no current stack.
+     * @return Stack|null Return null there is no current stack.
      */
     public function getCurrentStack()
     {
-        return end($this->data['stacks']);
+        if (false === $stack = end($this->data['stacks'])) {
+            return null;
+        }
+
+        return $stack;
     }
 
     /**

Collector/ProfileClient.php

@@ -124,7 +124,7 @@ public function sendRequest(RequestInterface $request)
      */
     private function collectRequestInformations(RequestInterface $request, Stack $stack = null)
     {
-        if (!$stack) {
+        if (null === $stack) {
             return;
         }
 
@@ -143,7 +143,7 @@ private function collectRequestInformations(RequestInterface $request, Stack $st
      */
     private function collectResponseInformations(ResponseInterface $response, StopwatchEvent $event, Stack $stack = null)
     {
-        if (!$stack) {
+        if (null === $stack) {
             return;
         }
 
@@ -163,7 +163,7 @@ private function collectExceptionInformations(\Exception $exception, StopwatchEv
             $this->collectResponseInformations($exception->getResponse(), $event, $stack);
         }
 
-        if (!$stack) {
+        if (null === $stack) {
             return;
         }
 

Collector/ProfilePlugin.php

@@ -58,7 +58,8 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
     {
         $profile = new Profile($this->pluginName, $this->formatter->formatRequest($request));
 
-        if ($stack = $this->collector->getCurrentStack()) {
+        $stack = $this->collector->getCurrentStack();
+        if (null !== $stack) {
             $stack->addProfile($profile);
         }