Fix psalm errors.

Author: markstory

psalm.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="5"
+    resolveFromConfigFile="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+</psalm>

src/Plugin.php

@@ -265,6 +265,6 @@ public function getPrimaryNamespace(PackageInterface $package)
             );
         }
 
-        return trim($primaryNs, '\\');
+        return trim((string)$primaryNs, '\\');
     }
 }