[Console][#18619] Prevent fatal error when calling Command#getHelper() without helperSet

Use Command::setHelperSet rather than Command#setHelperSet in exception msg

Simplify exception message

Add DidYouForget to exception msg

src/Symfony/Component/Console/Command/Command.php

@@ -537,10 +537,15 @@ class Command
      *
      * @return mixed The helper value
      *
+     * @throws \LogicException           if no HelperSet is defined
      * @throws \InvalidArgumentException if the helper is not defined
      */
     public function getHelper($name)
     {
+        if (null === $this->helperSet) {
+            throw new \LogicException(sprintf('Cannot retrieve helper "%s" because there is no HelperSet defined. Did you forget to add your command to the application or to set the application on the command using the setApplication() method? You can also set the HelperSet directly using the setHelperSet() method.', $name));
+        }
+
         return $this->helperSet->get($name);
     }
 

src/Symfony/Component/Console/Tests/Command/CommandTest.php

@@ -173,6 +173,16 @@ class CommandTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($formatterHelper->getName(), $command->getHelper('formatter')->getName(), '->getHelper() returns the correct helper');
     }
 
+    /**
+     * @expectedException        \LogicException
+     * @expectedExceptionMessage Cannot retrieve helper "formatter" because there is no HelperSet defined.
+     */
+    public function testGetHelperWithoutHelperSet()
+    {
+        $command = new \TestCommand();
+        $command->getHelper('formatter');
+    }
+
     public function testMergeApplicationDefinition()
     {
         $application1 = new Application();