[Tex/LaTex] Titlesec Nested titles error

documentclass-writingsectioningtitlesec

I am trying to write a class file (.cls) for my degree report following a university standard. However, when I try to edit the headings (for section, subsection or subsubsection) I get a nesting error.
This is my test program: 

% !TeX program = lualatex
\documentclass[]{exjobb}

\usepackage[english]{babel}
% Title Page
\title{A testing Typographithing}
\author{Tesla Testing}


\begin{document}
    \maketitle
Well this is just for testing
\begin{abstract}

text text
\end{abstract}

\section{SectionTitle}
section text

\end{document}

And this is my .cls file:

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{exjobb}[2017/03/18 A class based on format requirements for degree project report at the Xbio program att Uppsala University]
\LoadClass{article}
\RequirePackage{titlesec}
\RequirePackage{fontspec}
\RequirePackage{parskip}
\RequirePackage{setspace}



%% Set bread text options
\setmainfont{Times New Roman}
\setstretch{1.5}

\setlength{\parindent}{0pt}
\setlength{\parskip}{12pt}


%%%Set Header 1 options
\newfontface\headingfont{Arial}
%\titlelabel{\section.\quad}
\titleformat{name=\section}[display]{\Large\headingfont}{\section.\quad}{0em}{}{}

Through the process of elimination I understand that the problem occurs when \titleformat is introduced, but I don't understand why.

Best Answer

The section number is obtained with \thesection.

\titleformat{name=\section}[display]
  {\Large\headingfont}
  {\thesection.\quad}
  {0em}
  {}
  {}
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