Why does the message "The object reference was not set to an object instance" appear?

Solution:

The email client must have all the defaults in the control panel.

Follow these steps for Windows 10 (older operating systems may differ):

1. Open the Control Panel
2. Open the "Default Programs
3. Click on "Set default programs"
4. For the default "Email" app, select your email client. It may also help to temporarily select a different email client and then switch back to the correct one.
5. After that, scroll down and click on "Set default settings by app".
6. Locate your email client, click on it and click on "Manage".

Finally, select your email client from the list of possible file type and protocol mappings for each option.

If these steps do not help, check the registry. Please use the following program for this purpose:
https://csmftp.c4b.de/_webtools/tools/GetDefaultMailClient.exe 

Explanation: The program checks the registry in the same way as the client. The following four results are possible:

• Outlook: Entry for Outlook found
• Notes: Entry for Notes found
• Unknown: Entry found, but does not correspond to Outlook / Notes (e.g., Thunderbird)
• None: No entry found

Alternatively, you can also manually check the following locations in the registry:

HKEY_CURRENT_USER\Software\Clients\Mail
HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail

The name of the default email client is located to the right of the “(Default)” key. If the desired email client is not listed there, replace the value with the correct email client. To do this, use the name of the respective subkey from, for example, HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail as independent keys, e.g., “Microsoft Outlook” or “IBM Notes.”

If this does not solve the problem, check whether the Microsoft Office installation on the affected workstations was still 32-bit at the time of the XPhone Client installation and has now been changed to Microsoft Office 64-bit (or vice versa, if applicable), possibly in the course of an update to a newer Office version. If this is the case, perform a repair installation for XPhone Connect Client. This is necessary because the Microsoft Office platform is determined dynamically during the installation (32- or 64-bit) and thus the appropriate components are installed. During a repair installation, this detection is run again and the installation is corrected accordingly.