MergeableLineMarkerInfo: removing assert on nullable element in LineMarkerInfo

because actually it could be `null` if gutter outlasts the element it was put on (EA-119661)

IDEA-CR-32349

platform/lang-api/src/com/intellij/codeInsight/daemon/MergeableLineMarkerInfo.java

@@ -1,6 +1,7 @@
 // Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package com.intellij.codeInsight.daemon;
 
+import com.intellij.ide.IdeBundle;
 import com.intellij.openapi.editor.markup.GutterIconRenderer;
 import com.intellij.openapi.ui.popup.IPopupChooserBuilder;
 import com.intellij.openapi.ui.popup.JBPopupFactory;
@@ -144,11 +145,16 @@ public abstract class MergeableLineMarkerInfo<T extends PsiElement> extends Line
             icon = renderer.getIcon();
           }
           PsiElement element = ((LineMarkerInfo)dom).getElement();
-          assert element != null;
-          final String elementPresentation =
-            dom instanceof MergeableLineMarkerInfo
-            ? ((MergeableLineMarkerInfo)dom).getElementPresentation(element)
-            : element.getText();
+          final String elementPresentation;
+          if (element == null) {
+            elementPresentation = IdeBundle.message("node.structureview.invalid");
+          }
+          else if (dom instanceof MergeableLineMarkerInfo) {
+            elementPresentation = ((MergeableLineMarkerInfo)dom).getElementPresentation(element);
+          }
+          else {
+            elementPresentation = element.getText();
+          }
           String text = StringUtil.first(elementPresentation, 100, true).replace('\n', ' ');
 
           final JBLabel label = new JBLabel(text, icon, SwingConstants.LEFT);