Awt manual test AWT_DataTransfer/Manual/CustomTextFlavor fails

Testsuite name:AWT manual
Test name(s):AWT_DataTransfer/Manual/CustomTextFlavor
Product(s) tested:Oracle JDK 12+ea+25(64bit)
Test tool: tonga-4.0.2-rc2.jar
OS/architecture:ubuntu18.04-x64

Actual behavior:
1)The 'do it' button is disable.We try to switch right choice,'do it'button is disable also. Please refer to jdk12-Flavor.png.
2)On ubuntu18.04-x64 with JDK11.0.1b07(64bit):there is no such issue.Please refer to jdk11-Flavor.png.
3)On ubuntu18.10-x64 with JDK12b25(64bit):Failed the same issue.

Affect case:
AWT_DataTransfer/Manual/StringSelectionTextFlavor