private boolean validLibPath() {
		String extLibPath = getExtLibPath();
		if (extLibPath != null && extLibPath.length() > 0 &&
			!SystemUtilities.isEqual(autoDeterminedExternalLibraryPath, extLibPath)) {

			Project project = AppInfo.getActiveProject();
			ProjectData projectData = project.getProjectData();
			DomainFile file = projectData.getFile(extLibPath);
			if (file == null) {
				showInputErr("Cannot find the program for the specified library 'Path' of " +
					extLibPath + ".");
				return false;
			}
		}
		return true;
	}
Pop up the data tree dialog so the user can choose the external program.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/symboltree/EditExternalLocationPanel.java