Sketcher: Fix arc double click bug (#23873)

src/Mod/Sketcher/Gui/DrawSketchHandlerArc.h

@@ -398,6 +398,9 @@ private:
             // Prevent validation of null arc.
             return false;
         }
+        if (state() == SelectMode::SeekThird && fabs(arcAngle) < Precision::Confusion()) {
+            return false;
+        }
         return true;
     }
 
@@ -892,12 +895,6 @@ void DSHArcController::addConstraints()
         }
     }
     else {  // Valid diagnosis. Must check which constraints may be added.
-
-        // if no curve exists a crash occurs #12755
-        if (firstCurve < 0) {
-            return;
-        }
-
         auto startpointinfo = handler->getPointInfo(GeoElementId(firstCurve, pos1));
 
         if (x0set && startpointinfo.isXDoF()) {