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
if (i == 0)
                            extent = w - adjustFirst;
                        else
                            extent = w - adjust;
                        StyledText text = (StyledText) texts.get(i);
                        Point p = text.computeSize(extent, SWT.DEFAULT, false);
                        ((GridData) text.getLayoutData()).widthHint = p.x;
                    }
                     
                    
                    Point p = infoArea.computeSize(SWT.DEFAULT, SWT.DEFAULT,
                            true);
                    scrolledComposite.setMinHeight(p.y);
                }
            });
        }
        
        if (sampleStyledText == null) {
            this.scrolledComposite.getHorizontalBar().setIncrement(
                    HORZ_SCROLL_INCREMENT);
            this.scrolledComposite.getVerticalBar().setIncrement(
                    VERT_SCROLL_INCREMENT);
        } else {
            GC gc = new GC(sampleStyledText);
            int width = gc.getFontMetrics().getAverageCharWidth();
            gc.dispose();
            this.scrolledComposite.getHorizontalBar().setIncrement(width);
            this.scrolledComposite.getVerticalBar().setIncrement(
                    sampleStyledText.getLineHeight());
        }