Bug 329930 Resetting a perspective while its active part is not defined in the perspective causes the perspective to not have an active part after the reset
1. Window > Show View > Console 2. Window > Reset Perspective... > OK 3. No parts are active.
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/PartActivationHistory.java
if (parent == null && element instanceof MWindow) {
			
			parent = (MUIElement) ((EObject) element).eContainer();
		}
		if (parent == null) {
			return isValid(partService.getLocalPlaceholder(element));
		}
		return isValid(parent);
	}
	/**
	 * Checks to see if this element and its parents are actually being rendered.
	 */
	boolean isValid(MPerspective perspective, MUIElement element) {
		if (element instanceof MApplication) {
			return true;
		} else if (element == null || !element.isToBeRendered() || !element.isVisible()) {
			return false;
		}
		MElementContainer<?> parent = element.getParent();
		if (parent == null) {
			for (MPlaceholder placeholder : modelService.findElements(perspective, null,
					MPlaceholder.class, null)) {
				if (placeholder.getRef() == element) {
					return isValid(perspective, placeholder);
				}
			}
			if (element instanceof MWindow) {
				
				return isValid(perspective, (MUIElement) ((EObject) element).eContainer());
			}