Fix styling

Author: simonhamp

src/Dialog.php

@@ -110,7 +110,7 @@ public function properties(array $properties): self
         return $this;
     }
 
-    public function asSheet(string $windowId = null): self
+    public function asSheet(?string $windowId = null): self
     {
         if (is_null($windowId)) {
             $this->windowReference = Window::current()->id;

src/Menu/Menu.php

@@ -66,17 +66,17 @@ public function label(string $label): self
         return $this->add(new Label($label));
     }
 
-    public function checkbox(string $label, bool $checked = false, string $hotkey = null): self
+    public function checkbox(string $label, bool $checked = false, ?string $hotkey = null): self
     {
         return $this->add(new Checkbox($label, $checked, $hotkey));
     }
 
-    public function event(string $event, string $text, string $hotkey = null): self
+    public function event(string $event, string $text, ?string $hotkey = null): self
     {
         return $this->add(new Event($event, $text, $hotkey));
     }
 
-    public function link(string $url, string $text, string $hotkey = null): self
+    public function link(string $url, string $text, ?string $hotkey = null): self
     {
         return $this->add(new Link($url, $text, $hotkey));
     }

src/System.php

@@ -42,7 +42,7 @@ public function printers(): array
         })->toArray();
     }
 
-    public function print(string $html, Printer $printer = null): void
+    public function print(string $html, ?Printer $printer = null): void
     {
         $this->client->post('system/print', [
             'html' => $html,