Make forced line break in title of text widget

You can work around that by passing widget_title through a filter :

function custom_widget_title( $title ) {
    $title = str_replace( '__br__', '<br/>', $title );
    return $title;
}    
add_filter( 'widget_title', 'custom_widget_title' );

Now use __br__ in your widget title instead of <br/> and the code will replace it.

Note: You need to put the code in your theme’s functions.php file.