Bug 117925 [Intro] Misleading exception message in WelcomeEditor.init()
... it says it expects an IFileEditorInput, but it actually checks for WelcomeEditorInput. Suggested patch appended.
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/ide/dialogs/WelcomeEditor.java
copyAction.setEnabled(false);
    }
    /**
     * Update the welcome page to start at the
     * beginning of the text.
     */
    private void focusOn(StyledText newText, int caretOffset) {
        if (newText == null)
            return;
        newText.setFocus();
        newText.setCaretOffset(caretOffset);
        scrolledComposite.setOrigin(0, newText.getLocation().y);
    }
    /**
     * Finds the next text 
     */
    private StyledText nextText(StyledText text) {
        int index = 0;
        if (text == null)
            return (StyledText) texts.get(0);
        else
            index = texts.indexOf(text);
        
        if (index < texts.size() - 1)
            return (StyledText) texts.get(index + 1);
        else
            return (StyledText) texts.get(0);
    }
    /**
     * Finds the previous text