Update assertion leading to "Cannot substitute type on NULL reference"

Change-Id: Idc06a3b94f226dd734c497ddb12f5cd8fabc5674
diff --git a/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java b/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java
index a02bf6c..8c40d92 100644
--- a/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java
+++ b/src/main/java/com/android/tools/r8/ir/conversion/LensCodeRewriter.java
@@ -465,6 +465,7 @@
           assert !affectedPhis.contains(operandPhi) || operandType.isBottom();
           assert affectedPhis.contains(operandPhi)
               || operandType.isPrimitive()
+              || operandType.isNullType()
               || (operandType.isReference()
                   && operandType.asReferenceTypeLatticeElement().substitute(graphLense, appView)
                       == operandType);