Check if Mode is a str

If you initialize the gateway without parameters, mode is *not* a string
but instead an Enum, causing a "no such method" error when checking for the
":force" option.

Author: elevans

src/imagej/__init__.py

@@ -1207,7 +1207,10 @@ def init(
 
         ij = imagej.init("sc.fiji:fiji", mode=imagej.Mode.GUI)
     """
-    force = mode.endswith(":force")
+    force = False
+    if isinstance(mode, str):
+        force = mode.endswith(":force")
+
     if force:
         mode = mode[:-6]