Double clicking on Data Stores and Main Data Store just expands the list. Tested on clean, newly created project.
CUBA Studio 2018.3 (IntelliJ Edition)
Build #CS-183.10.1, built on June 7, 2019
JRE: 1.8.0_152-release-1343-b28 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Linux 5.2.1-arch1-1-ARCH