fix: IncomingRequest::getJsonVar() may cause TypeError

See #5391

[TypeError]
dot_array_search(): Argument #2 ($array) must be of type array, null given, called in .../system/HTTP/IncomingRequest.php on line 540
at SYSTEMPATH/Helpers/array_helper.php:21

Author: kenjis

system/HTTP/IncomingRequest.php

@@ -537,7 +537,11 @@ public function getJsonVar(string $index, bool $assoc = false, ?int $filter = nu
     {
         helper('array');
 
-        $data = dot_array_search($index, $this->getJSON(true));
+        $json = $this->getJSON(true);
+        if (! is_array($json)) {
+            return null;
+        }
+        $data = dot_array_search($index, $json);
 
         if ($data === null) {
             return null;

tests/system/HTTP/IncomingRequestTest.php

@@ -394,6 +394,29 @@ public function testGetVarWorksWithJsonAndGetParams()
         $this->assertSame('buzz', $all['fizz']);
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/issues/5391
+     */
+    public function testGetJsonVarReturnsNullFromNullBody()
+    {
+        $config          = new App();
+        $config->baseURL = 'http://example.com/';
+        $json            = null;
+        $request         = new IncomingRequest($config, new URI(), $json, new UserAgent());
+
+        $this->assertNull($request->getJsonVar('myKey'));
+    }
+
+    public function testgetJSONReturnsNullFromNullBody()
+    {
+        $config          = new App();
+        $config->baseURL = 'http://example.com/';
+        $json            = null;
+        $request         = new IncomingRequest($config, new URI(), $json, new UserAgent());
+
+        $this->assertNull($request->getJSON());
+    }
+
     public function testCanGrabGetRawInput()
     {
         $rawstring = 'username=admin001&role=administrator&usepass=0';