The TextMesh Pro UPM package is now included by default with Unity 2018.2. As such, if your project still contains the previous version of TextMesh Pro, you should be getting several errors in the console which can lead to the behavior you are reporting.
If you haven’t already done so, you will need to remove the older version of TextMesh Pro from your project by deleting the “TextMesh Pro” folder. Be sure to backup your project first as well as any files or assets you may have saved inside the TextMesh Pro folder hierarchy. See the following post with further details. Note that you will need to convert your project to the new format used by the new TextMesh Pro package which is also detailed in this post.
Since all packages installed via the Unity Package Manager located in the “Window - Package Manager” menu use Assembly Definitions, I strongly recommend you upgrade to the latest version of Visual Studio 2017 if you are using Visual Studio.
Let me know if after following the information from the referenced post and here, you still have the issues you reported.