Fix: Property can be null (#226)

* Enhancement: Assert that Segment throws SegmentException without initialization

* Fix: Property can be null

lib/Segment.php

@@ -6,7 +6,7 @@
 
 class Segment
 {
-    private static Client $client;
+    private static ?Client $client = null;
 
     /**
      * Initializes the default client to use. Uses the libcurl consumer by default.

test/SegmentTest.php

@@ -0,0 +1,78 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Segment\Test;
+
+use PHPUnit\Framework;
+use Segment\Segment;
+use Segment\SegmentException;
+
+final class SegmentTest extends Framework\TestCase
+{
+    protected function setUp(): void
+    {
+        self::resetSegment();
+    }
+
+    public function testAliasThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::alias([]);
+    }
+    public function testFlushThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::flush();
+    }
+    public function testGroupThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::group([]);
+    }
+    public function testIdentifyThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::identify([]);
+    }
+    public function testPageThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::page([]);
+    }
+    public function testScreenThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::screen([]);
+    }
+    public function testTrackThrowsSegmentExceptionWhenClientHasNotBeenInitialized(): void
+    {
+        $this->expectException(SegmentException::class);
+        $this->expectExceptionMessage('Segment::init() must be called before any other tracking method.');
+
+        Segment::track([]);
+    }
+
+    private static function resetSegment(): void
+    {
+        $property = new \ReflectionProperty(
+            Segment::class,
+            'client'
+        );
+
+        $property->setAccessible(true);
+        $property->setValue(null);
+    }
+}