Fix issues when inspecting renderer and volume profile assets under specific circumstances

This PR fixes two issues:

https://jira.unity3d.com/browse/UUM-71897 
Regression from recent PR https://github.cds.internal.unity3d.com/unity/unity/pull/48923 caused errors to be printed if URP is not the active pipeline when inspecting URP renderer assets. Added a null check.

https://jira.unity3d.com/browse/UUM-71890
When opening the editor, it's possible SRP is configured to be used but it has not been activated. In this situation you may see an empty volume inspector. Adding a message to let user know what is happening.

Author: arttu-peltonen

Packages/com.unity.render-pipelines.core/Editor/Volume/VolumeProfileEditor.cs

@@ -43,10 +43,13 @@ void OnDisable()
         /// <inheritdoc/>
         public override void OnInspectorGUI()
         {
-            if (componentList == null)
+            if (componentList == null || componentList.asset == null)
             {
                 if (!VolumeManager.instance.isInitialized)
+                {
+                    EditorGUILayout.HelpBox("Volume Profiles require an active Scriptable Render Pipeline, but nothing has been rendered. Make sure Scene or Game View is in focus and no debug modes are active.", MessageType.Warning);
                     return; // Defer initialization until VolumeManager is initialized
+                }
 
                 Init();
             }

Packages/com.unity.render-pipelines.universal/Editor/UniversalRendererDataEditor.cs

@@ -203,8 +203,8 @@ public override void OnInspectorGUI()
 
 
             EditorGUI.indentLevel--;
-            var renderGraphSettings = GraphicsSettings.GetRenderPipelineSettings<RenderGraphSettings>();
-            if (renderGraphSettings.enableRenderCompatibilityMode)
+            if (GraphicsSettings.TryGetRenderPipelineSettings<RenderGraphSettings>(out var renderGraphSettings)
+                && renderGraphSettings.enableRenderCompatibilityMode)
             {
                 EditorGUILayout.Space();
                 EditorGUILayout.LabelField(Styles.RenderPassSectionLabel, EditorStyles.boldLabel);