searchform in front page returns fatal error

I put the following searchform in my front page:

<form id="searchform" method="get" action="<?php bloginfo('home'); ?>">
<input type="text" name="s" id="s" size="25" />&nbsp;
<input type="submit" value="<?php esc_attr_e('Search Postpositive'); ?>" /></form>

It works fine when the page is accessed from the index.php in www root (i.e. https://www.myblog.examle.com/index.php).

But if someone goes to

https://www.myblog.example.com/wp-content/themes/mytheme/index.php

I get a fatal error:

Call to undefined function: bloginfo()

I realize this is expected behavior, but is there a way to test for something (some variable?) when the /wp-content/themes/mytheme/index.php is called directly?

If so, I assume I could just omit the searchform (there are no other wp function calls on the page).