Use anchor for element to avoid working with invalid element (KT-22631)

Element in constructor can be invalidated when dumb mode begins/ends.
Working with invalid elements can produce nodes without icons and bad
presentation.

 #KT-22631 Fixed

idea/idea-analysis/src/org/jetbrains/kotlin/idea/structureView/KotlinStructureViewElement.kt

@@ -35,7 +35,7 @@ import kotlin.properties.ReadWriteProperty
 import kotlin.reflect.KProperty
 
 class KotlinStructureViewElement(
-    val element: NavigatablePsiElement,
+    element: NavigatablePsiElement,
     private val isInherited: Boolean = false
 ) : PsiTreeElementBase<NavigatablePsiElement>(element), Queryable {
 
@@ -74,6 +74,8 @@ class KotlinStructureViewElement(
     }
 
     override fun getChildrenBase(): Collection<StructureViewTreeElement> {
+        val element = element
+
         val children = when (element) {
             is KtFile -> element.declarations
             is KtClass -> element.getStructureDeclarations()
@@ -104,7 +106,10 @@ class KotlinStructureViewElement(
     private fun isPublic(descriptor: DeclarationDescriptor?) =
         (descriptor as? DeclarationDescriptorWithVisibility)?.visibility == Visibilities.PUBLIC
 
-    private fun countDescriptor(): DeclarationDescriptor? = when {
+    private fun countDescriptor(): DeclarationDescriptor? {
+        val element = element
+        return when {
+            element == null -> null
             !element.isValid -> null
             element !is KtDeclaration -> null
             element is KtAnonymousInitializer -> null
@@ -114,6 +119,7 @@ class KotlinStructureViewElement(
                 } else null
             }
         }
+    }
 }
 
 private class AssignableLazyProperty<in R, T : Any>(val init: () -> T) : ReadWriteProperty<R, T> {