"Missing signature" window appears when pressing "Import TMP Essentials" button in the TMP Importer window

Reproduction steps:
1. Create a new project
2. Create any TextMeshPro GameObject (GameObject > UI (Canvas))
3. Press the "Import TMP Essentials" button in the TMP Importer window
4. Observe the Editor

Actual result: A "Missing signature" window appears
Expected result: TMP Essentials are imported without any additional windows appearing

Reproducible with: 6000.6.0b1, 6000.7.0a1
Not reproducible with: 6000.0.76f1, 6000.3.18f1, 6000.5.1f1, 6000.6.0a7

Reproduced on: Windows 11
Not reproduced on: macOS Tahoe 26.5 (M4)

Note: Pressing "Import Anyway" button in the "Missing signature" window throws these errors:

EndLayoutGroup: BeginLayoutGroup must be called first.
UnityEngine.GUILayout:EndVertical ()
TMPro.TMP_PackageResourceImporter:OnGUI () (at ./Library/PackageCache/com.unity.ugui@9e8950a7655a/Runtime/TMP/TMP_PackageResourceImporter.cs:99)
TMPro.TMP_PackageResourceImporterWindow:OnGUI () (at ./Library/PackageCache/com.unity.ugui@9e8950a7655a/Runtime/TMP/TMP_PackageResourceImporter.cs:268)
UnityEditor.GUIView:ProcessEvent (UnityEngine.EntityId,intptr,bool&) 

EndLayoutGroup: BeginLayoutGroup must be called first. UnityEditor.GUIView:ProcessEvent (UnityEngine.EntityId,intptr,bool&)