Don't crash when findLayer is called with a nullptr

src/core/layertree/qgslayertreegroup.cpp

@@ -193,6 +193,9 @@ void QgsLayerTreeGroup::removeAllChildren()
 
 QgsLayerTreeLayer *QgsLayerTreeGroup::findLayer( QgsMapLayer *layer ) const
 {
+  if ( !layer )
+    return nullptr;
+
   return findLayer( layer->id() );
 }
 

tests/src/core/testqgslayertree.cpp

@@ -47,6 +47,7 @@ class TestQgsLayerTree : public QObject
     void testLegendSymbolRuleBased();
     void testResolveReferences();
     void testEmbeddedGroup();
+    void testFindLayer();
     void testLayerDeleted();
 
   private:
@@ -589,6 +590,29 @@ void TestQgsLayerTree::testEmbeddedGroup()
   }
 }
 
+void TestQgsLayerTree::testFindLayer()
+{
+  //new memory layer
+  QgsVectorLayer *vl = new QgsVectorLayer( QStringLiteral( "Point?field=col1:integer" ), QStringLiteral( "vl" ), QStringLiteral( "memory" ) );
+  QVERIFY( vl->isValid() );
+
+  QgsProject project;
+  project.addMapLayer( vl );
+
+  QgsLayerTree root;
+  QgsLayerTreeModel model( &root );
+
+  QVERIFY( !root.findLayer( vl->id() ) );
+  QVERIFY( !root.findLayer( nullptr ) );
+
+  root.addLayer( vl );
+
+  QCOMPARE( root.findLayer( vl->id() )->layer(), vl );
+  QCOMPARE( root.findLayer( vl )->layer(), vl );
+  QVERIFY( !root.findLayer( QStringLiteral( "xxx" ) ) );
+  QVERIFY( !root.findLayer( nullptr ) );
+}
+
 void TestQgsLayerTree::testLayerDeleted()
 {
   //new memory layer