Issue while saving data to excel file

[Enigma86d]

Hi,

I tried to use FastExcel in a mobile app project done using Xamarin forms. I installed version 3.0.2 via NuGet.
I tried several examples (with similar outcomes) but I ended up focusing on "Update Demo".
I tried running the Update Demo code on three different excel files and got the below outcome:

1. "test_2003.xls"
   I created this file by opening Excel 365 and clicking save as 97-2003 *.xls file (empty file).
   When I run the Update Demo code I get this error (I've put here only the first few lines):
   System.IO.InvalidDataException: Number of entries expected in End Of Central Directory does not correspond to number of entries in Central Directory.
   at System.IO.Compression.ZipArchive.ReadCentralDirectory () [0x0004d] in /Users/builder/jenkins/workspace/archive-mono/2019-10/android/release/external/corefx/src/System.IO.Compression/src/System/IO/Compression/ZipArchive.cs:517
2. "test_365.xlsx"
   I created this file using Office 365 and saving it simply as a xlsx file (empty file).
   When I run the Update Demo code I don't get any errors but when I open the file I can't see any changes
3. "test.xlsx"
   This is a old xls file which I had on my computer prior to installing Office 365 so it is older than 365 but newer than xls files.
   The file contains just some text and links without any special stuff like text formatting, formulas, graphs, etc.
   After I run the Update Demo code and try to open the file, Excel says the file has error and after it tries to correct them it says "Replaced Part: /xl/worksheets/sheet1.xml part with XML error. Incorrect document syntax. Line 1, column 5773."

I must say that I am completely new to using FastExcel and also I just started making mobile apps just a year ago so it could be my mistake. However, after trying various things and thinking about it I am tempted to think there may be a bug with the library, hence I've made this post.

I have uploaded the files (both before and after running Update Demo - the ones that contain "original" are before running the code). Download link: https://wetransfer.com/downloads/3f1362963ede75b3fa62ec99767b593d20210111155642/a1ad2875e6e116f8049ff6b979aad57c20210111155656/9e046f

Kind regards,
Marius