Bug 300855 Do not allow Path Variables with empty location
HEAD I'm able to create a path variable without location. Next, when I want to edit the variable, Edit Variable dialog opens with error saying "You must provide a file or folder path as variable value.". I guess I should not be able to create such variable in the first place.
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/ide/dialogs/PathVariableDialog.java
Composite buttonsComposite = new Composite(contents, SWT.NONE);
        buttonsComposite.setLayoutData(new GridData(SWT.END, SWT.CENTER, false,
        		false, 1, 1));
        GridLayout layout = new GridLayout(3, true);
        layout.marginWidth = 0;
        layout.marginHeight = 0;
        buttonsComposite.setLayout(layout);
        
        fileButton = new Button(buttonsComposite, SWT.PUSH);
        fileButton.setText(IDEWorkbenchMessages.PathVariableDialog_file);
        fileButton.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, false,
        		false));
        if ((variableType & IResource.FILE) == 0) {
			fileButton.setEnabled(false);
		}
        fileButton.addSelectionListener(new SelectionAdapter() {
            public void widgetSelected(SelectionEvent e) {
                selectFile();
            }
        });
        
        folderButton = new Button(buttonsComposite, SWT.PUSH);
        folderButton.setText(IDEWorkbenchMessages.PathVariableDialog_folder);
        folderButton.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, false,
        		false));
        if ((variableType & IResource.FOLDER) == 0) {
			folderButton.setEnabled(false);
		}
        folderButton.addSelectionListener(new SelectionAdapter() {