private boolean validLocationAddress() {

		AddressSpace locationAddressSpace = extAddressInputWidget.getAddressSpace();
		if (locationAddressSpace != null) {
			if (extAddressInputWidget.hasInput()) {
				Address locationAddress = extAddressInputWidget.getAddress();
				if (locationAddress == null) {
					showInputErr("Invalid address specified, " + locationAddressSpace.getName() +
						" offset must be in range: " +
						locationAddressSpace.getMinAddress().toString(false) + " to " +
						locationAddressSpace.getMaxAddress().toString(false));
					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