Unity 2018.2 already includes a version of the TMP UPM package. As such, if you have a previous version of the source code or dll-only version of TextMesh Pro, you end up with two versions of TextMesh Pro in the same project.
In this case, you will have to upgrade to the new TextMesh Pro Unity Package Manager (UPM) version. Please see the following sticky post with detailed instructions.
To summarize these steps
(1) Backup your project
(2) Close all open scenes.
(3) Remove previous version of TextMesh Pro by deleting the “TextMesh Pro” folder. Be sure to review any changes you may have made to the TMP Settings or to any of the assets contained in the “TextMesh Pro” folder hierarchy.
(4) Install the latest version of the TMP UPM package by going to “Window - Package Manager” and selecting version 1.2.4 (which is the latest).
(5) Import the TMP Essential Resources as well as the TMP Examples and Extras (optional but recommended). See “Window - TextMeshPro - …” menu.
(6) Convert project files (related to TMP) to the new set of GUIDs and FileIDs used by the new TMP UPM package. This is done by using the “Project Files GUID Remapping Tool” also found in “Window - TextMeshPro - …” menu.
At this point everything should be working as expected. If you run into any issue, please let me know.
Note about the conversion: For most users this scanning process is pretty quick but there have been cases where it took hours. I would like to make this faster, but I am still trying to get my hands on one of those projects. Everything I have tested thus far takes a few minutes to convert.
The migration to the new TMP UPM package and conversion of the project to the new format can be a pain but once running on these new TMP UPM versions, subsequent upgrades are so much simpler via the package manager.