removed default constructor of FXDialogService

[r0light]

Fix for #4840. Needs test and discussion

Removed injection mechanism for DialogService in order to remove its default constructor, because through the null value used in the default constructor, the NullPointerExceptions were caused.

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• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[tobiasdiez]

Please don't remove the injection. Instead of a new FXDialogService you can here return JabRefGUI.getMainFrame().getDialogService().

[Siedlerchr]

Yep, please don't remove injection. This is very handy and makes it otherwise very complicated

[r0light]

Ok, second try :) I reverted the removal of the injection. But it was not possible to simply call JabRefGUI.getMainFrame().getDialogService() inside DefaultInjector, because at the first time it is needed, JabRefFrame is not ready yet. In fact, from what i concluded, the injection is used somwhere inside the init-method of JabRefFrame which was called inside its constructor.
Therefore, i now splitted the constructor of JabRefFrame and the init-method, so that in JabRefGUI the object ist first created and then init is called. I'm not so happy with setting init public, but found no other way.. I also tried to use a factory method, but the constructed JabRefFrame-object is needed in JabRefGUI, so that JabRefGUI.getJabRefFrame().getDialogService()works.
Finally, i also removed the second DialogService in JabRefGUI, so that there is only a single DialogService.

[Siedlerchr]

And you need the JabRefFrame as container for the JFXSnackbar? So that was the initial reason for the sitting of the notify method in BasePanel.

[r0light]

Yes, well the reason for the output method in JabRefFrame. So i have to admit, i broke (some of) the notifications with #4815, but it was also confusing with the different methods and the deprecated method in JabRefFrame. So i hope, the code is a little better now.

[Siedlerchr]

looks better now

[Siedlerchr]

Some lines earlier you have JabRefGui.getMainFrame() instead of mainFrame

[r0light]

Yes, it was incosistent, i used only mainFrame now in the whole class.

[tobiasdiez]

Thanks for the follow-up. The code looks great now!