bug: Abstract identification error when 'abstract' is not explicitly written

[MasterYip]

PS C:\Users\Steven\ChatPaper2Xmind> python paper2xmind.py --path "D:\123.pdf"
Traceback (most recent call last):
  File "C:\Users\Steven\ChatPaper2Xmind\paper2xmind.py", line 54, in <module>
    pdf_batch_processing(argparser.parse_args().path)
  File "C:\Users\Steven\ChatPaper2Xmind\paper2xmind.py", line 42, in pdf_batch_processing
    pdf_processing(pdf_path, xmind_path)
  File "C:\Users\Steven\ChatPaper2Xmind\paper2xmind.py", line 15, in pdf_processing
    paper = PDFPaperParser(pdf_file_path)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Steven\ChatPaper2Xmind\pdf_parser.py", line 23, in __init__
    self.section_textdict = self.get_section_textdict()
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Steven\ChatPaper2Xmind\pdf_parser.py", line 182, in get_section_textdict
    section_title = self.get_section_titles(withlevel=False)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Steven\ChatPaper2Xmind\pdf_parser.py", line 158, in get_section_titles
    section_title = [(re.search(ABS_MATCHSTR, self.all_text).group(), 1)]\
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'group'

TODO: Robust abstract detection mechanism & error raising with clear explanation