Dropdown menu warning in https page

Hello David!
I have faced a minor problem with MLA when I switched my site into secure protocol (https). Now when I choose a certain category in dropdown menu, the browser always warns me “The information you have entered on this page will be sent over an insecure connection and could be read by a third party. Are you sure you want to send this information?” and I always have to answer “yes” before I get to the desired category page.
Is there a way to avoid this annoying question?

The code of the page is:

<div style="margin:0.5em 0 0.5em 1em">
[mla_term_list]  
mla_markup=dynamic-dropdown
mla_style=term-list-my-dropdown
mla_item_parameter=current_attachment_category 
option_all_text='Выбрать категорию' 
option_all_value=all
show_count=true 
pad_counts=false 
hierarchical=true
exclude_tree=68,200,154,127,273
[/mla_term_list]
</div>

<div style="clear:both"></div>

<h1 style="margin: 0.5em 0 0.5em 0.75em; text-align: left; padding: 0px;"><span style="letter-spacing:1px;text-transform: none;font-size:0.8em">Текущая категория: </span>?[mla_gallery ids=1] mla_nolink_text="{+template:({+current_term:attachment_category+}|Demo)+}"[/mla_gallery]?</h1>

<div class="gallery-description">
<div style="heigt:50px;width:50px; float:left; margin: 1em 0;"></div>

[mla_gallery]
mla_nolink_text="{+template:({+current_term:attachment_category(description)+}| Иллюстрации к статьям Back To Photo — исследование тонких свойств старого фотооборудования и переделка фототехники и оптики с учетом Векторной Направленности компонентов. Особенности рисунка фотографий, снятых экспериментальным фотооборудованием можно оценить отсортировав их по категориям и стилям, отличия наиболее заметны при просмотре в полноэкранном режиме.)+}"
[/mla_gallery]
</div>

[mla_gallery] style='random' tile_size='large' slide_size='full' caption='title-desc' headers='caption,counter'
attachment_category="{+template:({+request:attachment_category,raw+}|{+request:tax_input.attachment_category,raw+}|demo)+}"
posts_per_page=50
link=file
mla_caption="{+caption+} : {+description+}"
mla_nolink_text="<br> <br>"
orderby="ID DESC"
mla_page_parameter="pg"
[/mla_gallery]

[mla_gallery]
attachment_category="{+template:({+request:attachment_category,raw+}|{+request:tax_input.attachment_category,raw+}|demo)+}"
posts_per_page=50
mla_output="paginate_links"
mla_link_href='{+new_url+}'
mla_page_parameter="pg"
[/mla_gallery]

The page I need help with: [log in to see the link]