Fix crash on array access to undefined class

src/Psalm/Internal/Analyzer/Statements/Expression/Fetch/ArrayFetchAnalyzer.php

@@ -1737,8 +1737,10 @@ private static function handleArrayAccessOnNamedObject(
         ?Union &$array_access_type,
         bool &$has_array_access
     ): void {
+        $codebase = $statements_analyzer->getCodebase();
         if (strtolower($type->value) === 'simplexmlelement'
-            || $statements_analyzer->getCodebase()->classExtendsOrImplements($type->value, 'SimpleXMLElement')
+            || ($codebase->classExists($type->value)
+                && $codebase->classExtendsOrImplements($type->value, 'SimpleXMLElement'))
         ) {
             $call_array_access_type = new Union([new TNull(), new TNamedObject('SimpleXMLElement')]);
         } elseif (strtolower($type->value) === 'domnodelist' && $stmt->dim) {

tests/ArrayAccessTest.php

@@ -1199,6 +1199,27 @@ function f($p): int {
                 ',
                 'assertions' => ['$a===' => "array{1: 'b'}"],
             ],
+            'noCrashOnUnknownClassArrayAccess' => [
+                'code' => <<<'PHP'
+                <?php
+
+                namespace Psalmtest\Psalmtest;
+
+                use SomeMissingClass;
+
+                class Test
+                {
+                    public function f(): void {
+                        /** @var SomeMissingClass */
+                        $result = null;
+
+                        if ($result['errors'] === true) {}
+                    }
+                }
+                PHP,
+                'assertions' => [],
+                'ignored_issues' => ['UndefinedDocblockClass'],
+            ],
         ];
     }