To reproduce:
1. Open any project along with ProBuilder
2. Open ProBuilder's UV Editor (Tools>Probuidler>Editors>Open UV Editor)
3. Create any ProBuilder shape
4. In Game View, switch to Face Selection mode
5. Select any face, and while holding ctrl, try selecting any outer edge of the object (check video for clearer explanation)

Expected: another face is added to multi-selection
Actual: object has new material applied (any shades of black)

Reproduced in: 2018.1.2f1, 2018.2.0b6

Note1: sometimes the color being applied differs
Note2: only tested on releases with integrated ProBuilder due to older releases being obsolete
Note3: full error:
"NullReferenceException: Object reference not set to an instance of an object
ProBuilder.MeshOperations.pb_UVOps.AutoStitch (ProBuilder.Core.pb_Object pb, ProBuilder.Core.pb_Face f1, ProBuilder.Core.pb_Face f2) (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Classes/ClassesEditing/pb_UVOps.cs:299)
ProBuilder.EditorCore.pb_UVEditor.ClickShortcutCheck (ProBuilder.Core.pb_Object pb, ProBuilder.Core.pb_Face selectedFace) (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/pb_UVEditor.cs:806)
ProBuilder.EditorCore.pb_Editor.RaycastCheck (Vector3 mousePosition, Int32 deepClickOffset) (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/pb_Editor.cs:855)
ProBuilder.EditorCore.pb_Editor.OnSceneGUI (UnityEditor.SceneView sceneView) (at C:/Users/karlh/procore/probuilder/probuilder2.0/Assets/ProCore/ProBuilder/Editor/EditorCore/pb_Editor.cs:609)
UnityEditor.SceneView.CallOnSceneGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:2607)
UnityEditor.SceneView.HandleSelectionAndOnSceneGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1909)
UnityEditor.SceneView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneView.cs:1746)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:291)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:284)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:251)"