Removed a race_condition

 - fixes #5126
 - `view->buffer` from `RenderJob` could be nullptr. when deleted by a different thread

src/core/control/jobs/RenderJob.cpp

@@ -22,6 +22,12 @@
 #include "view/DocumentView.h"          // for DocumentView
 #include "view/Mask.h"                  // for Mask
 
+#if defined(__has_cpp_attribute) && __has_cpp_attribute(likely)
+#define XOJ_CPP20_UNLIKELY [[unlikely]]
+#else
+#define XOJ_CPP20_UNLIKELY
+#endif
+
 using xoj::util::Rectangle;
 
 RenderJob::RenderJob(XojPageView* view): view(view) {}
@@ -45,8 +51,11 @@ void RenderJob::rerenderRectangle(Rectangle<double> const& rect) {
     renderToBuffer(newMask.get());
 
     std::lock_guard lock(this->view->drawingMutex);
-    assert(view->buffer.isInitialized());
-
+    if (!view->buffer.isInitialized()) {
+        // Todo: the buffer must not be uninitializable here, either by moving it into the job or by locking it at job
+        // creation a shared prt may also be suffice.
+        XOJ_CPP20_UNLIKELY return;
+    }
     newMask.paintTo(view->buffer.get());
 }