[escher/expression_field] Fixed dumpLayout

Closing the Calculation app because of a full pool would crash the
device, by attempting to dump a layout with no node.

Change-Id: Ic926da61ae93f47eb576848788e1de5e3f94f5bb

escher/include/escher/expression_view.h

@@ -26,6 +26,7 @@ public:
   KDSize minimalSizeForOptimalDisplay() const override;
   KDPoint drawingOrigin() const;
   KDPoint absoluteDrawingOrigin() const;
+  bool layoutHasNode() const { return Poincare::TreeNode::IsValidIdentifier(m_layout.identifier()) && !m_layout.wasErasedByException(); }
 protected:
   /* Warning: we do not need to delete the previous expression layout when
    * deleting object or setting a new expression layout. Indeed, the expression

escher/include/escher/layout_field.h

@@ -32,6 +32,7 @@ public:
   }
   bool hasText() const { return layout().hasText(); }
   Poincare::Layout layout() const { return m_contentView.expressionView()->layout(); }
+  bool layoutHasNode() const { return m_contentView.expressionView()->layoutHasNode(); }
   CodePoint XNTCodePoint(CodePoint defaultXNTCodePoint) override;
   void putCursorRightOfLayout();
   void setInsertionCursorEvent(Ion::Events::Event event) { m_insertionCursorEvent = event; }

escher/src/expression_field.cpp

@@ -136,6 +136,16 @@ size_t ExpressionField::dumpLayout(char * buffer, size_t bufferSize) const {
      * has changed and invalidate the data.*/
     returnValue = 0;
   } else {
+    /* dumpLayout will be called whenever Calculation exits, event when an
+     * exception occurs. We thus need to check the validity of the layout we
+     * are dumping (m_layoutField.m_contentView.m_expressionView.m_layout).
+     * However, attempting to get a handle on a layout that has been erased
+     * will crash the program. We need the check to be performed on the
+     * original object in expressionView. */
+    if (!m_layoutField.layoutHasNode()) {
+      buffer[0] = 0;
+      return 0;
+    }
     size = m_layoutField.layout().size();
     currentLayout = reinterpret_cast<char *>(m_layoutField.layout().node());
     returnValue = size;