Bibliography error in main TEX file

Question

I keep getting the following error when I try and Build and View/Compile my main TEX file:

\begin{thebibliography} on input line 1 ended by \end{document}. \end{document}

This is a new error, and I cannot work out why it is occurring. My bibliography has been functioning as expected until recently. Below is the code for my main TEX file, which uses a .bib file called "References.bib" which can be seen below the TEX file code. I have cut out a lot of the chapters in a bid to give the question more clarity, but I saw it appropriate to include all of the preamble. It is the very last line of this file "\end{document}" that shows the error.

The main TEX file called "thesis.tex":

\documentclass{report}

\usepackage{gensymb}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{natbib}
\AtBeginDocument{\renewcommand{\bibname}{References}}
\usepackage{graphicx}
\usepackage[utf8]{inputenc}
\graphicspath{ {images/} }
\usepackage[a4paper,width=150mm,top=25mm,bottom=25mm]{geometry}
\usepackage{fancyhdr}
\pagestyle{fancy}
\usepackage{titlesec}
\usepackage{pgfornament}
\titleformat{\chapter}[display]
{\normalfont\bfseries}{}{0pt}{\Huge}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{nomencl}
\makenomenclature

\begin{document}

\chapter{Conclusion}
\chaptermark{}
\input{chapters/conclusion}

\addcontentsline{toc}{chapter}{References}
\bibliographystyle{agsm}
\bibliography{References}

\appendix
\chapter{Appendix}
\chaptermark{}
\input{chapters/appendix}

\chapter{Project Management}
\chaptermark{}
\input{chapters/management}

\chapter{Self- Review}
\chaptermark{}
\input{chapters/review}

\end{document}

Here is the contents of the "References.bib" file:

    @conference{Xconference1,
    author    = "Avinash Gautam and Sudeept Mohan",
    title     = "A review of research in multi-robot systems",
    booktitle = " Industrial and Information Systems (ICIIS)",
    year      = "2012",
    month    = "August",
    publisher= "IEEE",
}

@misc{ultrasonic_diagram,
    author = "Senix",
    title = "How-Do-Ultrasonic-Sensors-Work.png",
    note = "Online; accessed May 07, 2018",
    url = "https://senix.com/frequently-asked-questions/",
    %year = "",
}

@misc{iRobot,
    author = "Army-Technology",
    title = "iRobot 510 PackBot Multi-Mission Robot",
    note = "Online; accessed September 12, 2017",
    url = "http://www.army-technology.com/projects/irobot-510-packbot-multi-mission-robot/
    Coastguard National Database",
    %year = "",
}

@misc{analog_v_digital,
    author = "Tarun Agarwal",
    title = "Types of Analog and Digital Sensors with Applications",
    note = "Online; accessed May 08, 2018",
    url = "https://www.elprocus.com/types-analog-digital-sensors/",
    %year = "",
}

@misc{sensorscite,
    author = "P.S Kang",
    title = "01 introduce to lego ev3 20140917.",
    note = "Online; accessed December 12 2017",
    url = "https://www.slideshare.net/JasonYang35/01-introduce-to-lego-ev3-20140917",
    year = "2015",
}

@misc{polling,
    author = "Linus Atorf",
    title = "How accurate is the ultrasound sensor when moving?",
    note = "Online; accessed May 08 2018",
    url = "https://bricks.stackexchange.com/questions/1257/how-accurate-is-the-ultrasound-sensor-when-moving/7",
    year = "2012",
}

If anyone could help me figure out why this is happening, I'd be very appreciative. I have tried to add the following code to the References.bib file, but it did not work:

\begin{thebibliography}

     %Some references 

\end{thebibliography}