Problem
The plugin was not compatible with JetBrains Rider 2026.1 due to breaking changes in the SDK and related packages.
Changes made

Updated JetBrains.Rider.SDK to 2026.1.0.1
Updated Gradle wrapper to 9.4.1
Updated intellij-platform Gradle plugin to 2.5.0
Updated Kotlin to 2.1.10
Fixed SubmenuBehavior usage — constructor became private, replaced with SubmenuBehavior.Custom()
Removed [NotNull]/[CanBeNull] annotations from JetBrains.Annotations which became inaccessible in the new SDK
Fixed deprecated toLowerCase() → lowercase() in protocol/build.gradle.kts
Fixed project.exec {} → providers.exec {} for Gradle 9 compatibility
Fixed Rider dependency declaration: rider(version) { useInstaller = false }
Fixed platform type from "riderRD" to "RD"

Tested on

Rider 2026.1
.NET Framework 4.7.2