fix: empty textbox double click causes fatal error

src/graphics/ui/elements/TextBox.cpp

@@ -337,6 +337,9 @@ inline std::wstring get_alphabet(wchar_t c) {
 }
 
 void TextBox::tokenSelectAt(int index) {
+    if (input.empty()) {
+        return;
+    }
     int left = index;
     int right = index;