This is wrong:
add_theme_page("Theme Panel", "Theme Panel", "manage_options", "theme-panel", "initialize_theme_options", null, 99);
The last two parameters shouldn’t be there. Check this function in codex here.
EDIT:
What’s more, the last parameter (which should be there) initialize_theme_options is wrong too, because it should be a callback to a function which would output a content. So for example your main_settings_callback function.
SECOND EDIT:
It doesn’t work you probably because in the call of add_settings_section function you pass in the last parameter theme-panel and this page probably doesn’t exist. Am I right? Try to replace this parameter for example with general and it’ll output at the main Setting page.